Introduce play-cycle-complete?

[vemv]

It can be hard to ensure play* never is running simultaneously from multiple threads (which would result in scrambled sound output).

The following wouldn't work:

(while @*playing* 
  (Thread/sleep default-sleep-duration))

...because it is possible that *playing* is set to false and yet play* is still engaged in IO operations:

clj-audio/src/clj_audio/core.clj

Lines 157 to 162 in b245047

	(if (and (> cnt -1) @*playing*)
	(do
	(when (> cnt 0)
	(.write source buffer 0 cnt))
	(recur (.read audio-stream buffer 0 (alength buffer))
	(+ total cnt)))

In the snippet above, .write and .read are blocking calls, so probably you can see there's a chance for race conditions.

This PR introduces a finer-grained variable, so one can:

(while (or @*playing* (not @play-cycle-complete?))
  (Thread/sleep default-sleep-duration))

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Of course, one could prevent concurrent calls from the consumer side, but it also seems nice to provide a generic opt-in mechanism.