MB-12350 DCP info message contains misleading statistics.

Add a new parameter to backfill received which contains information to the source
of the backfill (memory vs disk). Increment stat counters based upon this
data as the current stat accounting was always counting disk.

Change-Id: I5ab0602540deedad536e78517850393594341b32
Reviewed-on: http://review.couchbase.org/42153
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>