Issue 352: Change SubscriberAsReactiveSubscriber to properly apply back-pressure

[alexandru]

The SubscriberAsReactiveSubscriber conversion does not work properly for async Subscribers, because in version 2.2.4 it uses an unbounded buffer and it keeps requesting from the source without listening to the Future[Ack] given by the downstream subscriber. The communication in this case is OK, since a buffer is being used, but the source is not paused while the target is busy.

This PR fixes the implementation to apply back-pressure using when communicating with the Reactive Streams Publisher, in accordance with the response given by the downstream Monix Subscriber.

Other changes:

• add TrampolinedExecutionContext.immediate reference, used as an optimization
• adds tests for this back-pressure change to guard against regressions
• Observable.fromReactivePublisher gets an overload that can specify the requestCount on each batch
• the bufferSize param in that call chain gets renamed to requestCount, because the actual buffer in the implementation remains Unbounded (yet its size will be controlled by requestCount due to back-pressure)

/cc @jchapuis

[codecov]

Codecov Report

Merging #361 into master will decrease coverage by 0.03%.
The diff coverage is 66%.

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
- Coverage   86.32%   86.28%   -0.04%     
==========================================
  Files         313      313              
  Lines        8378     8409      +31     
  Branches     1662     1679      +17     
==========================================
+ Hits         7232     7256      +24     
- Misses       1146     1153       +7

[codecov]

Codecov Report

Merging #361 into master will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
+ Coverage   86.32%   86.35%   +0.02%     
==========================================
  Files         313      313              
  Lines        8378     8411      +33     
  Branches     1662     1664       +2     
==========================================
+ Hits         7232     7263      +31     
- Misses       1146     1148       +2

[jchapuis]

great! I can see this triggered quite a good number of changes. Also the additional coverage looks awesome, since effects of backpressure and cancelation are hard to observe from the outside.