Issues to be fixed with the latest operators

[akarnokd]

While merging the new operators from issue #1060, some open questions remained. This issue tracks the fixes for those.

• OperatorMulticast to not subscribe/unsubscribe while holding the lock.
• OperatorRefCount to not connect/disconnect while holding the lock
• OperatorSampleWithTime to unsubscribe when onComplete is called so the periodic schedule can be stopped.