MDEV-225: Replace with dummy events an event that is not understood by a slave to which it should be sent

Add function to replace arbitrary event with dummy event.

Add code which uses this to fix the bug that enabling row_annotate events
on the master breaks slaves which do not request such events.

Add that slaves set a variable @mariadb_slave_capability to inform the
master in a robust way about which events it can, and cannot, handle.

Add tests.