Work around deadlock problems with VACUUM FULL/CLUSTER on system catalogs,

Work around deadlock problems with VACUUM FULL/CLUSTER on system catalogs,
as per my recent proposal.

First, teach IndexBuildHeapScan to not wait for INSERT_IN_PROGRESS or
DELETE_IN_PROGRESS tuples to commit unless the index build is checking
uniqueness/exclusion constraints.  If it isn't, there's no harm in just
indexing the in-doubt tuple.

Second, modify VACUUM FULL/CLUSTER to suppress reverifying
uniqueness/exclusion constraint properties while rebuilding indexes of
the target relation.  This is reasonable because these commands aren't
meant to deal with corrupted-data situations.  Constraint properties
will still be rechecked when an index is rebuilt by a REINDEX command.

This gets us out of the problem that new-style VACUUM FULL would often
wait for other transactions while holding exclusive lock on a system
catalog, leading to probable deadlock because those other transactions
need to look at the catalogs too.  Although the real ultimate cause of
the problem is a debatable choice to release locks early after modifying
system catalogs, changing that choice would require pretty serious
analysis and is not something to be undertaken lightly or on a tight
schedule.  The present patch fixes the problem in a fairly reasonable
way and should also improve the speed of VACUUM FULL/CLUSTER a little bit.