Implement review comments from Robert and Mickael.

Namely that VACUUM and CLUSTER shouldn't grow support for ddl_command_*
events in this patch, and the use of both in_table_rewrite and tableOid
rather than a single table_rewrite_oid that is InvalidOid when unused.
Revert the CLUSTER and VACUUM changes.
Adjust the docs and tests for the lack of CLUSTER and VACUUM support.
Bugfix pg_event_trigger_table_rewrite_oid().
Review and simplify ALTER TABLE support, add more tests.
Move the invoke call, per comments from Alvaro.
Uppercase SQL keywords in docs examples.