In some cases we can get rid of MULTI/EXEC operations too.
- putDocInMemory: from 10 down to 2 operations
- removeDocFromMemory: from 14+4 down to 4+4 operations
- updateDoc: from 13 down to 8 operations
Doc contents are added only after the tracking has been setup.
All read paths on the tracking have been checked to gracefully handle
the case of existing doc_id but missing doc contents.
- getDoc: -1 operation
REF: 0a2b47c660c60b95e360d8f3b3e30b862ceb6d79