Do not update relay-log.info and master.info on disk after every event
when using GTID mode:

 - relay-log.info and master.info are not crash-safe, and are not used
   when slave restarts in GTID mode (slave connects with GTID position
   instead and immediately rewrites the file with the new, correct
   information found).

 - When using GTID and parallel replication, the position in
   relay-log.info is misleading at best and simply wrong at worst.

 - When using parallel replication, the fact that every single
   transaction needs to do a write() syscall to the same file is
   likely to become a serious bottleneck.

The files are still written at normal slave stop.

In non-GTID mode, the files are written as normal (this is needed to
be able to restart after slave crash, even if such restart is then not
crash-safe, no change).