The worklog creates the infrastructure enabling two new
timestamps that are associated with each transaction
(instead of event or statement) in the binary log:
- original_commit_timestamp: microseconds since the epoch
when the transaction was committed on the original master.
- immediate_commit_timestamp: microseconds since the epoch
when the transaction was committed on the immediate master.

Summary of relevant points covered by this worklog:

1) Two new fields, original_commit_timestamp and
immediate_commit_timestamp, added to gtid_log_event. Commit
time is recorded in write_gtid(), so the commit timestamp
of a transaction is assumed to be the time of writing it to
the binary log.
2) has_commit_timestamps flag added to the Gtid_event
class, indicating whether the current gtid_log_event
includes at least the immediate_commit_timestamp. If the
gtid_log_event does not include any commit timestamp, it
was replicated from a server with a MySQL version lower
than 8.0 and has to be processed accordingly.
4) The new body of gtid_log_event consists of:
 - only original_commit_timestamp at the original server,
 since original and immediate commit timestamps are same at
 this server;
 - both original and immediate commit timestamps in any
 other 8.0+ server.