MB-20182: Update checkpoint snapshot correctly during TAP backfill

MB-20182: Update checkpoint snapshot correctly during TAP backfill

When we do a TAP backfill we must update checkpoint snapshot start
and end correctly. Otherwise, if we immediately proceed to DCP
after TAP backfill, the checkpoint mgr will have incorrect combination
of {snap_start, snap_end, vb_high_seqno}

Change-Id: I2b738fd3b24486dadbd2962e81e0c3820c5a8786
Reviewed-on: http://review.couchbase.org/65866
Tested-by: buildbot <build@couchbase.com>
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Manu Dhundi <manu@couchbase.com>