Binlog events are read in a stream pipeline/chain way, like:
Byte Stream --> Event Data Stream --> Event Object Stream
- Event data is the serialized binlog event. It is a chunk of data in buffer.
  Event data istream read data from byte stream and organize byte data into
  event data.
- Event object stream trasforms event data to Log_event object.

Byte Stream includes:
- Binlog_ifile
  Read binlog files generated on master side
- Relaylog_ifile
  Read binlog files generated on slave side
- Mysqlbinlog_ifile
  Read both master and slave side binlog files through os files or stdin.

Event Data Stream includes:
- Binlog_event_data_istream
  Event_data is serialized event object. It is a chunk of data in buffer.
  Binlog_event_data_istream fetches byte data from Basic_istream and
  divides them into event_data chunk according to the format.
- Mysqlbinlog_event_data_istream
  Handle a couple of special things from mysqlbinlog

Event Object Stream includes:
- Binlog_event_object_istream
  It read event_data from an event data stream and deserialize the event_datas
  to Log_event object.

* Reader classes
  To make code simple and easy to use the pipeline is encapsulate in to a
  template class called Basic_binlog_file_reader.

  It owns a byte stream, an event_data stream and an event object stream.
  The stream pipeline is setup in constructor. All the objects required for
  reading a binlog file is initialized in reader class. It also includes
  some convenient functions to encapsulate the access of the streams.

  Reader classes have:
  - Binlog_file_reader
  - Relaylog_file_reader
  - Binlog_sender::File_reader
  - Mysqlbinlog_file_reader