This patch replaced the last Log_event::read_log_event() call to
Relaylog_file_reader which was introduced in Step2 patch. So
Log_event::read_log_event() was finally removed from the code.

It also did some refacotring
- It created Rpl_applier_reader class which is responsible for reading
  events from relaylog files.
- The code of next_event was moved into Rpl_applier_reader class and
  separated into a few functions.
- The applier reader is only need when applier is online. It is used
  as a local variable of handle_slave_sql().
- change_master and purge_relay_logs don't need to reopen or reinitialize
  the applier read anymore. So rli::init_relay_log_pos is reorganized as
  group_relay_log_is_valid() to verify group_relay_log_name existing.
- rli::event_relay_log_pos and rli::event_relay_log_name are only used
  when applying events. So they just need to be intilaized in Rpl_applier_reader
  class.
- Moved the code period checking out mts_checkpoint_routine. It was
  encapsulated into a new function called rli::is_time_for_mts_checkpoint