rxjava2-pool - resolve MemberSingle race conditions - move observers …

…modification to the drain loop (#148)

Concurrency unit testing revealed occasional failures that were due to race conditions with modification of observers outside the MemberSingle.drain loop. To resolve this all modification actions to the observers happen via queues that are polled in the drain loop.

With the simplifications and extra tests, coverage has increased a bit. Unlikely to put in the hard tedious yards to knock off the rest.

This is a fix backported from https://github.com/davidmoten/rxjava3-pool.