Subject state information methods & bounded ReplaySubject termination

[akarnokd]

fix.

This PR aims to support the requests in #2331 and #1897 by adding methods to the (final) subject classes that let developers access in-flight state in a snapshot fashion.

This may be considered safe API change because all subject classes were final already so adding extra methods won't break anyone's code (and we will be careful with our Observable in the future).

• NotificationLite is now extended with two missing value checks: isNull and isNext.
• Each subject has hasCompleted, hasThrowable and getThrowable methods, however, I can't add them to Subject because that would be an incompatible API change.
• Where applicable, getValue and getThrowable return null instead of throwing exceptions so users are encouraged to call hasXXX methods beforehand.
• There was a chaining bug in the bounded ReplaySubject: because the terminal value was added after a potential pruning, the node links could get broken and concurrent replays might not have seen the terminal value.
• Since ReplaySubject can have multiple values, I've added size(), hasAnyValue() (isEmpty is taken) and getValues() methods to make a snapshot of the current buffer contents whether or not the ReplaySubject has terminated (the usual toList() would wait until the subject has terminated).
• The unrelated OperatorMergeTest.testConcurrency hangs for me for some reason without activity (either a buffer bug or a merge bug is in play there). I've added a timeout so it doesn't stop the other tests.

[akarnokd]

I'm merging this: changes are either internal or annotated with experimental on final classes.