This ticket is only to start writing the timestamp to fast_count_metadata_store as field "valid-as-of" or "lastValid" every time ReplicatedFastCountManager flushes. The in-memory store still just tracks one timestamp, which is the last commitTimestamp for each collection, and to update it every time a write commits. All APIs and structures should be updated accordingly too.

The timestamp will not be used anywhere yet as part of this ticket.