fix DumpSchema for MySQL and CockroachDB

[alnr]

mysqldump would previously try to connect through a Unix socket if the host (-h) flag was given as localhost. This change forces a TCP connection. Connecting through a Unix socket is still supported.

cockroach sql would previously always connect on the default port. This change uses the actual DSN information to dump the schema. I've also removed the useless comments from the output.