MB-9307: Find header in block size increments

To find a header, don't scan the file byte by byte, but
only on block boundaries where the headers are stored.

Change-Id: Ia6fd86bc0b707db723b86b4d76488ba0d2b78b51
Reviewed-on: http://review.couchbase.org/30019
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>