MB-11725: Add log message to help find cause of dataloss

There are two potential cases that might be the root cause of data
loss. One is that the parameters supplied to the backfill task are
incorrect and the other is that the backfill is actually done
improperly somewhere in couchstore. This log message will help us
narrow down where the issue is.

Change-Id: I31fd8c001ac220287fb2072b94aef95bede77da1
Reviewed-on: http://review.couchbase.org/39642
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
Remove log messages that were used ot diagnose a warmup issue

These messages were added temporarily and are no longer neccessary
to keep in the code base.

Change-Id: I63ea88e3755ce2e63c35e629ccad1eff096cbe53
Reviewed-on: http://review.couchbase.org/39643
Reviewed-by: Sriram Ganesan <sriram@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
MB-11721: Send acks for snapshot markers during the takeover phase

By sending acks for snapshot markers we can make sure that all items
hve actually been processed on the new master before sending any
set vbucket state messages. This change will reduce the time that
a vbucket might be unavailable due to state changes.

Change-Id: I70b64035c4684754bfcdb632f29cb83fecf161a1
snap start/end seqno stuff in memory only

Change-Id: I7c9430e0b24ef01cc427cb03b518f4c41a55d875