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: I573ecdd32576c01a648c931dc8d0dd7c6e58906c
Reviewed-on: http://review.couchbase.org/29031
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>