pgsql: Fix race conditions when an event trigger is added concurrently

Fix race conditions when an event trigger is added concurrently with DDL.

EventTriggerTableRewrite crashed if there were table_rewrite triggers
present, but there had not been when the calling command started.

EventTriggerDDLCommandEnd called ddl_command_end triggers if present,
even if there had been no such triggers when the calling command started,
which would lead to a failure in pg_event_trigger_ddl_commands.

In both cases, fix by doing nothing; it's better to wait till the next
command when things will be properly initialized.

In passing, remove an elog(DEBUG1) call that might have seemed interesting
four years ago but surely isn't today.

We found this because of intermittent failures in the buildfarm. Thanks
to Alvaro Herrera and Andrew Gierth for analysis.

Back-patch to 9.5; some of this code exists before that, but the specific
hazards we need to guard against don't.

Discussion: https://postgr.es/m/5767.1523995174@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b1b71f16581fb5385fa9f9a663ffee271cdfaba5

Modified Files
--------------
src/backend/commands/event_trigger.c | 28 +++++++++++++++++++++++++---
1 file changed, 25 insertions(+), 3 deletions(-)