Make a shortcut for raw mode file:write_file.

And use it throughout. This prevents file:write_file going through
file_server_2.

Change-Id: Ib5c375d70de6ca40a5e9b75c6fb7bbcee7ef35a7
Reviewed-on: http://review.couchbase.org/38504
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Add behaviour annotation.

Change-Id: I6186fe7c546bffe8cec200832d16eb6f15a4c46b
Reviewed-on: http://review.couchbase.org/38505
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>