Facilities for async programming and communication.

go blocks are dispatched over an internal thread pool, which
defaults to 8 threads. The size of this pool can be modified using
the Java system property `clojure.core.async.pool-size`.

Set Java system property `clojure.core.async.go-checking` to true
to validate go blocks do not invoke core.async blocking operations.
Property is read once, at namespace load time. Recommended for use
primarily during development. Invalid blocking calls will throw in
go block threads - use Thread.setDefaultUncaughtExceptionHandler()
to catch and handle.

Use the Java system property `clojure.core.async.executor-factory`
to specify a function that will provide ExecutorServices for
application-wide use by core.async in lieu of its defaults. The
property value should name a fully qualified var. The function
will be passed a keyword indicating the context of use of the
executor, and should return either an ExecutorService, or nil to
use the default. Results per keyword will be cached and used for
the remainder of the application. Possible context arguments are:

:io - used in async/io-thread, for :io workloads in flow/process,
and for dispatch handling if no explicit dispatch handler is
provided (see below)

:mixed - used by async/thread and for :mixed workloads in
flow/process

:compute - used for :compute workloads in flow/process

:core-async-dispatch - used for completion fn handling (e.g. in put!
and take!, as well as go block IOC thunk processing) throughout
core.async. If not supplied the ExecutorService for :io will be
used instead.

The set of contexts may grow in the future so the function should
return nil for unexpected contexts.