implement PG ranges/multiranges agnostically

implement PG ranges/multiranges agnostically

Ranges now work using a new Range object,
multiranges as lists of Range objects (this is what
asyncpg does.  not sure why psycopg has a "Multirange"
type).

psycopg, psycopg2, and asyncpg are currently supported.
It's not clear how to make ranges work with pg8000, likely
needs string conversion; this is straightforward with the
new archicture and can be added later.

Fixes: #8178
Change-Id: Iab8d8382873d5c14199adbe3f09fd0dc17e2b9f1