Investigate supporting the `:named` paramstyle for `comdb2.dbapi2`

Comdb2 now supports this parameter style, and it's easier to use (and doesn't require escaping `%` characters in the query itself). Investigate whether there's a migration path to recommending this parameter style by default. It would be ideal if we could find a way to support both parameter styles simultaneously and detect which is being used, but failing that we could consider a parameter to `connect` to control which style is used.