CockroachDB support

>CockroachDB is a cloud-native SQL database for building global, scalable cloud services that survive disasters.

>CockroachDB supports the PostgreSQL wire protocol, so you can use any available PostgreSQL client drivers to connect from various languages.

go-imap-sql currenlty doesn't work with CockroachDB due to the following known problems:
* GROUP BY behaves differently
https://github.com/cockroachdb/cockroach/issues/26709

There are probably more problems, documented above are the only known ones.

go-imap-sql fully supports PostgreSQL and I would prefer to wait for CockroachDB to become more compatible with it than to introduce various hacks.

CC @NamedKitten