- Removed double call to trans_begin() for GTID BEGIN event
- Don't set OPTION_BEGIN before calling trans_begin() as this causes extra work in trans_begin()

sql/log_event.cc:
  Removed double call to trans_begin for GTID BEGIN event.
  Don't set OPTION_BEGIN before calling trans_begin() as this causes extra work in trans_begin().
  This was done by removing parsing of "BEGIN" and instead executing trans_begin() direct.
  This is much faster, but we lost the ability logging of possible slow "BEGIN" statements. As this should never happen it can be ignored.