MB-9307: Add find_header_bin/2

Make it possible to search for a header from a given position
backwards.

Change-Id: Id8254d2cdcce298fff964d576563ccb7383a461d
Reviewed-on: http://review.couchbase.org/29731
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>