Event Trigger for table_rewrite

Event Trigger for table_rewrite

Generate a table_rewrite event when ALTER TABLE
attempts to rewrite a table. Provide helper
functions to identify table and reason.

Intended use case is to help assess or to react
to schema changes that might hold exclusive locks
for long periods.

Dimitri Fontaine, triggering an edit by Simon Riggs

Reviewed in detail by Michael Paquier