Incidental cleanup of matviews code.

Move checking for unscannable matviews into ExecOpenScanRelation, which is
a better place for it first because the open relation is already available
(saving a relcache lookup cycle), and second because this eliminates the
problem of telling the difference between rangetable entries that will or
will not be scanned by the query.  In particular we can get rid of the
not-terribly-well-thought-out-or-implemented isResultRel field that the
initial matviews patch added to RangeTblEntry.

Also get rid of entirely unnecessary scannability check in the rewriter,
and a bogus decision about whether RefreshMatViewStmt requires a parse-time
snapshot.

catversion bump due to removal of a RangeTblEntry field, which changes
stored rules.
pg_dump: Improve message formatting
Fix unsafe event-trigger coding in ProcessUtility().

We mustn't run any of the event-trigger support code when handling
utility statements like START TRANSACTION or ABORT, because that code
may need to refresh event-trigger cache data, which requires being
inside a valid transaction.  (This mistake explains the consistent
build failures exhibited by the CLOBBER_CACHE_ALWAYS buildfarm members,
as well as some irreproducible failures on other members.)

The least messy fix seems to be to break standard_ProcessUtility into two
functions, one that handles all the statements not supported by event
triggers, and one that contains the event-trigger support code and handles
the statements that are supported by event triggers.

This change also fixes several inconsistencies, such as four cases where
support had been installed for "ddl_event_start" but not "ddl_event_end"
triggers, plus the fact that InvokeDDLCommandEventTriggersIfSupported()
paid no mind to isCompleteQuery.

Dimitri Fontaine and Tom Lane