MB-11488: Check memory usage before upr-backfill

Snooze the upr-backfill task if memory usage at that
point is found to be greater than the threshold.

Change-Id: I05aa930854a72bf6016f1138ec705ebff7af6f54
Reviewed-on: http://review.couchbase.org/38758
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>
MB-11459: Properly set high seqno in couchkvstore

We missed two places where we should update the cached version of
the vbucket high seqno.

Change-Id: I0b7e32348948814cb54d93feb5d9d78c8d478196
Reviewed-on: http://review.couchbase.org/38774
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
Add more logging

Change-Id: Id82d7c7569e37143a0413a90b54331452c8b2502