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.