Add a OnManualFlushScheduled callback in event listener

[jowlyzhang]

As titled. Also added the newest user-defined timestamp into the MemTableInfo. This can be a useful info in the callback.

Added some unit tests as examples for how users can use two separate approaches to allow manual flush / manual compactions to go through when the user-defined timestamps in memtable only feature is enabled. One approach relies on selectively increase cutoff timestamp in OnMemtableSeal callback when it's initiated by a manual flush. Another approach is to increase cutoff timestamp in OnManualFlushScheduled callback. The caveats of the approaches are also documented in the unit test.