Add support for unix socket connection

[bcardiff]

Before this PR connection strings were usually in the form mysql://root@localhost/test

After this PR that still works by we allow

• to specify database as a query parameter mysql://root@localhost?database=test
• to use unix sockets when uri don't have a host (there are 3 /) mysql:///path/to/mysql.sock?database=test. initial database always need to be a query param for unix socket.
• user/pass can still be specified as mysql://user:pass@/path/to/mysql.sock

This is a breaking change if you relied on manual setup the connection MySql::Connection::Options has changed a bit. That's the only breaking change in this PR regarding public API.

The CI will now use native mysql because there is no way to use socket via docker AFAIK. And mysql 5.6 is dropped from CI because some small amount of specs where failing, but also EOL.

The goal is to keep working on more transport options that will enable better mysql 8.0 support.

Closes #63
Closes #94