Add an example of attaching a default value to an updatable view.

This is probably the single most useful thing that ALTER VIEW can do,
particularly now that we have auto-updatable views.  So show an explicit
example.
Reduce log level of picksplit-doesn't-support-secondary-split whining.

This was agreed to back in 2007, but never actually done.

Josh Hansen
WIP for scannable state tracking.

Most of what was suggested by Noah Misch in included, although
the handling of zero-length heap segment is not done.
Merge branch 'master' of /home/kgrittn/pg/postgresql into matview