Fix to speed up Sieve processing, taking relatively a lot of time that

Fix to speed up Sieve processing, taking relatively a lot of time that
would cause heavy load if there are lots of users with lots of scripts.
rewrite_ctdl_sieve_config() was rewriting the sieve config every time
regardless of the yes_write_to_disk flag thus calling quickie_message()
which takes a bit of time (an extra 0.3mS).