Followup work from PR #1939

[ScottDugas]

Here are some comments that hinted at a desire for some followup work:

• [ ] https://github.com/FoundationDB/fdb-record-layer/pull/1939#discussion_r1060787030
• [ ] https://github.com/FoundationDB/fdb-record-layer/pull/1939#discussion_r1060806843
• [ ] https://github.com/FoundationDB/fdb-record-layer/pull/1939#discussion_r1060922591
• [ ] https://github.com/FoundationDB/fdb-record-layer/pull/1939#discussion_r1060967316

[jjezra]

To do/check/consider list

• Should synced multi target apply sync lock for all the targets?
• Stop indexing upon typeStamp change
• • add API stop block/unblock indexing process by changing the typestamp
• Is there any way to test store.getPrimaryKeyBoundaries?
• Share common indexer tests code in OnlineIndexerTest
• Allow mutual single index continue to a mutual multi target session?