Channel sockets for Clojure/Script.

    Protocol  | client>server | client>server ?+ ack/reply | server>user push
  * WebSockets:       ✓              [1]                           ✓
  * Ajax:            [2]              ✓                           [3]

  [1] Emulate with cb-uuid wrapping
  [2] Emulate with dummy-cb wrapping
  [3] Emulate with long-polling

Abbreviations:
  * chsk      - Channel socket (Sente's own pseudo "socket")
  * server-ch - Underlying web server's async channel that implement
                Sente's server channel interface
  * sch       - server-ch alias
  * uid       - User-id. An application-level user identifier used for async
                push. May have semantic meaning (e.g. username, email address),
                may not (e.g. client/random id) - app's discretion.
  * cb        - Callback
  * tout      - Timeout
  * ws        - WebSocket/s
  * pstr      - Packed string. Arbitrary Clojure data serialized as a
                string (e.g. edn) for client<->server comms
  * udt       - Unix timestamp (datetime long)

Special messages:
  * Callback wrapping: [<clj> <?cb-uuid>] for [1], [2]
  * Callback replies: :chsk/closed, :chsk/timeout, :chsk/error

  * Client-side events:
      [:chsk/ws-ping] ; ws-ping from server
      [:chsk/handshake [<?uid> nil[4] <?handshake-data> <first-handshake?>]]
      [:chsk/state     [<old-state-map> <new-state-map> <open-change?>]]
      [:chsk/recv      <ev-as-pushed-from-server>] ; Server>user push

  * Server-side events:
      [:chsk/ws-ping] ; ws-ping from client
      [:chsk/ws-pong] ; ws-pong from client
      [:chsk/uidport-open  <uid>]
      [:chsk/uidport-close <uid>]
      [:chsk/bad-package   <packed-str>]
      [:chsk/bad-event     <event>]

Channel socket state map:
  :type               - e/o #{:auto :ws :ajax}
  :open?              - Truthy iff chsk appears to be open (connected) now
  :ever-opened?       - Truthy iff chsk handshake has ever completed successfully
  :first-open?        - Truthy iff chsk just completed first successful handshake
  :uid                - User id provided by server on handshake,    or nil
  :handshake-data     - Arb user data provided by server on handshake
  :last-ws-error      - ?{:udt _ :ev <WebSocket-on-error-event>}
  :last-ws-close      - ?{:udt _ :ev <WebSocket-on-close-event>
                          :clean? _ :code _ :reason _}
  :last-close         - ?{:udt _ :reason _}, with reason e/o
                          #{nil :clean :unexpected :requested-disconnect
                            :requested-reconnect :downgrading-ws-to-ajax
                            :ws-ping-timeout :ws-error}
  :udt-next-reconnect - Approximate udt of next scheduled auto-reconnect attempt

Notable implementation details:
  * core.async is used liberally where brute-force core.async allows for
    significant implementation simplifications. We lean on core.async's
    efficiency here.
  * For WebSocket fallback we use long-polling rather than HTTP 1.1 streaming
    (chunked transfer encoding). Http-kit _does_ support chunked transfer
    encoding but a small minority of browsers &/or proxies do not. Instead of
    implementing all 3 modes (WebSockets, streaming, long-polling) - it seemed
    reasonable to focus on the two extremes (performance + compatibility).
    In any case client support for WebSockets is growing rapidly so fallback
    modes will become increasingly irrelevant while the extra simplicity will
    continue to pay dividends.

General-use notes:
  * Single HTTP req+session persists over entire chsk session but cannot
    modify sessions! Use standard a/sync HTTP Ring req/resp for logins, etc.
  * Easy to wrap standard HTTP Ring resps for transport over chsks. Prefer
    this approach to modifying handlers (better portability).

[4] Used to be a csrf-token. Was removed in v1.14 for security reasons.
A `nil` remains for limited backwards-compatibility with pre-v1.14 clients.

Alpha, subject to change.
Public interfaces / extension points.
Ref. https://github.com/ptaoussanis/sente/issues/425 for more info.

Alpha - subject to change!
Optional Transit-format[1] IPacker implementation for use with Sente.
[1] https://github.com/cognitect/transit-format.

Sente server adapter for Aleph,
Ref. <https://github.com/clj-commons/aleph>.

Sente server adapter for Node.js with Dog Fort,
Ref. <https://github.com/whamtet/dogfort>.

Sente server adapter for Node.js with Express,
Ref. <https://github.com/expressjs/express>.

This adapter works differently that the others as Sente is
expecting Ring requests but Express uses http.IncomingMessage.
While most of this adapter could be used for similar
implementations there will be assumptions here that the following
express middleware (or equivalents) are in place:
  - cookie-parser
  - body-parser
  - csurf
  - express-session
  - express-ws

See the example project at https://goo.gl/lnkiqS for an
implementation (it's a bit different than something built on Ring).

Sente server adapter for Node.js using the `ws` and `http` libraries.
Ref. <https://github.com/websockets/ws>,
     <https://nodejs.org/api/http.html>,
     <https://nodejs.org/en/docs/guides/anatomy-of-an-http-transaction>,
     <https://github.com/theasp/sente-nodejs-example>.

Sente server adapter for Immutant v2+,
Ref. <https://github.com/immutant/immutant>.

Sente server adapter for Node.js with the Macchiato Framework,
Ref. <https://github.com/macchiato-framework/macchiato-core>.

Sente server adapter for ring-undertow-adapter,
Ref. <https://github.com/luminus-framework/ring-undertow-adapter>.

Sente server adapter for http-kit,
Ref. <https://github.com/http-kit/http-kit>.