Clean up data conversion short-lived memory context.

dblink uses a short-lived data conversion memory context. However it
was not deleted when no longer needed, leading to a noticeable memory
leak under some circumstances. Plug the hole, along with minor
refactoring. Backpatch to 9.2 where the leak was introduced.

Report and initial patch by MauMau. Reviewed/modified slightly by
Tom Lane and me.
Add Asserts to verify that catalog cache keys are unique and not null.

The catcache code is effectively assuming this already, so let's insist
that the catalog and index are actually declared that way.

Having done that, the comments in indexing.h about non-unique indexes
not being used for catcaches are completely redundant not just mostly so;
and we didn't have such a comment for every such index anyway.  So let's
get rid of them.

Per discussion of whether we should identify primary keys for catalogs.
We might or might not take that further step, but this change in itself
will allow quicker detection of misdeclared catcaches, so it seems worth
doing in any case.
Fix documentation template for CREATE TRIGGER.

By using curly braces, the template had specified that one of
"NOT DEFERRABLE", "INITIALLY IMMEDIATE", or "INITIALLY DEFERRED"
was required on any CREATE TRIGGER statement, which is not
accurate.  Change to square brackets makes that optional.

Backpatch to 9.1, where the error was introduced.
9.4 release notes:  adjust some entry wording

Backpatch to 9.4
doc:  adjust JSONB GIN index description

Backpatch through 9.4
Fix bug in WAL_DEBUG.

The record header was not copied correctly to the buffer that was passed
to the rm_desc function. Broken by my rm_desc signature refactoring patch.
Add missing closing parenthesis into max_replication_slots doc.
Fix typo in replication slot function doc.
Improve tab-completion of DROP and ALTER ENABLE/DISABLE on triggers and rules.

At "DROP RULE/TRIGGER triggername ON ...", tab-complete tables that have
a rule/trigger with that name.

At "ALTER TABLE tablename ENABLE/DISABLE TRIGGER/RULE ...", tab-complete to
rules/triggers on that table. Previously, we would tab-complete to all
rules or triggers, not just those that are on that table.

Also, filter out internal RI triggers from the list. You can't DROP them,
and enabling/disabling them is such a rare (and dangerous) operation that
it seems better to hide them.

Andreas Karlsson, reviewed by Ian Barwick.
merge master