Protocol  | client>server | client>server ?+ ack/reply | server>user push

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

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

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
  :csrf-token         - CSRF token 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 :requested-disconnect :requested-reconnect
                           :downgrading-ws-to-ajax :unexpected}
  :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 semi-backwards-compatibility with pre-v1.14 clients.

Sente client adapter for Aleph (https://github.com/ztellman/aleph).

Alpha, subject to change.
Public interfaces / extension points.

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 (https://github.com/ztellman/aleph).