MB-8029 Avoid unnecessary endianness convertions

For temporary files, used only during indexing, don't care about
converting record lengths from native endian to big endian.

Change-Id: I5f8c2110863729075f4ee02d200e4ab172a2000d
Reviewed-on: http://review.couchbase.org/29034
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>