Remove Assert that matview is not in system schema from REFRESH.

We don't want to prevent an extension which creates a matview from
being installed in pg_catalog.

Issue was raised by Hitoshi Harada.
Backpatched to 9.3.
docs:  document TRIM "comma" syntax

This syntax is supported by the parser, but is non-standard.

_Not_ backpatched to 9.3 in case we change our minds.
Remove ph_may_need from PlaceHolderInfo, with attendant simplifications.

The planner logic that attempted to make a preliminary estimate of the
ph_needed levels for PlaceHolderVars seems to be completely broken by
lateral references.  Fortunately, the potential join order optimization
that this code supported seems to be of relatively little value in
practice; so let's just get rid of it rather than trying to fix it.

Getting rid of this allows fairly substantial simplifications in
placeholder.c, too, so planning in such cases should be a bit faster.

Issue noted while pursuing bugs reported by Jeremy Evans and Antonin
Houska, though this doesn't in itself fix either of their reported cases.
What this does do is prevent an Assert crash in the kind of query
illustrated by the added regression test.  (I'm not sure that the plan for
that query is stable enough across platforms to be usable as a regression
test output ... but we'll soon find out from the buildfarm.)

Back-patch to 9.3.  The problem case can't arise without LATERAL, so
no need to touch older branches.
Improve error message when view is not updatable

Avoid using the term "updatable" in confusing ways.  Suggest a trigger
first, before a rule.
Treat timeline IDs as unsigned in replication parser

Timeline IDs are unsigned ints everywhere, except the replication parser
treated them as signed ints.