GitLab

Fixed by caching last binary log number used in last_used_log_number

Other things:
- Moved locking of LOCK_log form new_file_impl() to new_file(). This fixed
  a bug where LOCK_log could have been unlocked even if 'need_lock' was
  not set.  Removed not anymore used argument need_lock.
- Made generate_new_name() virtual to simplify the code between
  other logs and binary log.

Reviewed by Andrei Elkin