COMMIT TRIGGER implementation using CONSTRAINT TRIGGERs

I've written an approximation of "commit triggers" for PostgreSQL using
CONSTRAINT TRIGGERs (as users have often been told to do who want this
feature).

Semantics (and a warning) are included in commentary in the source:

https://github.com/twosigma/postgresql-contrib/
https://github.com/twosigma/postgresql-contrib/blob/master/commit_trigger.sql
https://raw.githubusercontent.com/twosigma/postgresql-contrib/master/commit_trigger.sql

I suspect a number of users here may find this useful.

I've also posted separately to pgsql-hackers to see if we can get
agreement that commit triggers are a reasonable and desirable feature.

Cheers,

Nico
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