- Some deep-in-the-weeds fixes to try to get "server default" comparison

- Some deep-in-the-weeds fixes to try to get "server default" comparison
working better across platforms and expressions, in particular on
the Postgresql backend, mostly dealing with quoting/not quoting of various
expressions at the appropriate time and on a per-backend basis.
Repaired and tested support for such defaults as Postgresql interval
and array defaults.
fixes #212