the replication slave to be X seconds behind the master.

This entails the following changes:
- New syntax: CHANGE MASTER TO MASTER_DELAY = <int>
- The SQL thread sleeps as needed.
- SHOW SLAVE STATUS has three new fields with information about the
delay and the SQL thread status.
- When the slave SQL thread is sleeping, SHOW PROCESSLIST indicates this.
- RESET SLAVE resets the delay.
- The option is saved to the relay_log.info file.
- The BINLOG statement no longer accepts event types other than
Format_description_log_event or row events.

In addition, some small refactorings and bug fixes were needed to make
this work. See file-specific commit messages for details.