MB-7656 Allow max db header to be a parameter

Because the current hard coded value is only appropriate for
databases, and a couch_file_reader module should be generic
enough, not knowing anything specific about databases.

Change-Id: Ieec17af30817438dc402373509ac7bf3cac6b6f7
Reviewed-on: http://review.couchbase.org/28789
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>