CBIDXT-173 Process UPR Snapshot Markers

- Process UPR Snapshot Markers from Projector Messages.
  Timekeeper now stores vbuuid, snapshot markers as part of HWT.
- Refactored Timekeeper code a bit
- Don't Use pointers to Maps as maps are references anyway.

Change-Id: I6ff39cbd68bbacb51c074a126244f5372ea4e645
Reviewed-on: http://review.couchbase.org/41183
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>