rewrite OnSubscribeRefCount to handle synchronous source, add null check to OperatorMulticast
make fields final, use CompositeSubscription, unlock in finally clauses
abandon ReadWriteReentrantLock and simplify locking to a single lock that is used for subscription and disconnect
synchronize access to subscription for null check in OperatorMulticast
Merge pull request #1695 from davidmoten/refcount-1688

rewrite OnSubscribeRefCount to handle synchronous source
Issue #1642 Fix null-emitting combineLatest
Merge pull request #1761 from edenman/0.20.x

Issue #1642 Fix null-emitting combineLatest
Cleanup RefCount Tests
Merge pull request #1764 from benjchristensen/refCount-tests

Cleanup RefCount Tests
[Gradle Release Plugin] - pre tag commit:  '0.20.6'.
[Gradle Release Plugin] - new version commit:  '0.20.7-SNAPSHOT'.
0.20.6