rdb: fix consolidate when using user-referenced buffers

Previously we handled the logic wrong where we would mess up the
recvd.nused count. This would end up confusing lots of code.

Change-Id: Id77318e959075a000292b156e6fb70d0bbe189da
Reviewed-on: http://review.couchbase.org/38287
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>