Copied the code from OperationMerge
Merge pull request #615 from abersnaze/issue614

Copied the code from OperationMerge to fix synchronization
replay() and timer() javadocs: diagrams & wiki links
Merge pull request #617 from Netflix/docs

replay() and timer() javadocs: diagrams & wiki links
ObserveOn Fixes

- fix subscription leak (Composite+MultipleAssignment instead of just Composite)
- add unit tests
Merge pull request #602 from benjchristensen/observeOn

ObserveOn Fixes
UnitTest to assert thread safety of Executor Scheduler

- The Executor thread-pools hop threads when using observeOn so this tests that it is thread-safe when doing so.
- The test performs non-thread-safe mutations which should fail (or at least be non-deterministic) if memory visibility problems or concurrency is occurring.
Merge pull request #619 from benjchristensen/executorSchedulerConcurrencyTests

UnitTest to assert thread safety of Executor Scheduler