add support for multiple hosts in connection settings (for feature parity with libpq)

[costela]

Hi,

It would be nice to support a couple of new (>=10) libpq features, namely multiple hosts support and RW/RO connection selector. Both features are somewhat codependent.

This would mean adding support in the connstring parsing and probably refactoring conn.errRecover to retry connections (taking a cursory look).

If there's no objection on principle, I would give it a try.

[costela]

FYI: patch #714 has an attempt at adding this feature. I'd be grateful for reviews and/or testing.