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.

[ajkr]

The FlushRequest generated by FlushMemTable() specifies std::numeric_limits<uint64_t>::max() as the max memtable ID to persist. Is it possible the background flush includes memtables newer than the one in memtable_ids_to_wait? That would mean increasing the cutoff TS here would not guarantee the flush can happen.

[jowlyzhang]

That's a good question. Yes, if another memtable is sealed in between the IncreaseFullHistoryTsLow here and the time background flush checks for whether timestamps expired. It could mean that the cutoff TS here is not sufficient to guarantee the flush can happen.

If that sealed memtable is caused by another manual flush type of event, this call back should have also be invoked to increase the cutoff TS to a higher point. If that sealed memtable is caused by regular writes filling up a memtable, this would be an issue, like when the write rate is very high.

I think updating the memtable id in FlushRequest to be GetLatestMemTableID instead of std::numeric_limits<uint64_t>::max() can help make sure the flush can still proceed in this case. Do you have any concerns for making this change?