streams: parallel execution of independent sources with ordered output

[cportele]

Add two operators that let independent inner sources run concurrently without changing the order of the emitted items:

• Transformer.flatMapConcurrent(fn, maxConcurrency, prefetch): subscribes up to maxConcurrency mapped inner sources at once while emitting their items in the original order (concatMapEager).
• Source.guarded(acquire, release, inner): runs acquire on subscribe and release on termination on a worker thread, so a blocking acquire (e.g. taking permits from a semaphore that bounds resource use) does not block the subscribing thread.