First patch preparing for restructuring the event execution and event
skipping. This patch renames the existing (virtual) function exec_event()
to be a primitive for implementing the real patch. Also, the virtual function
advance_coord_impl() is added to advance the binary/relay log coordinates,
and two non-virtual functions exec_event() [sic] and skip_event() is added
that will contain the business logic for executing and skipping events
respectively.


sql/log.cc:
  Renaming exec_event()
sql/log_event.cc:
  Renaming exec_event()
sql/log_event.h:
  Renaming exec_event() to apply_event_impl()
  Addng new non-virtual exec_event() that will contain business logic
  for event execution.
  Adding new non-virtual skip_event() that will contain business logic
  for skipping an event.
  Adding new virtual advance_coord_impl() as primitive to advance
  binary/relay log coordinates according to the event's needs.