Support secure TLS connections

[clue]

Currently, the protocol parser only supports the plaintext MySQL protocol. The MySQL protocol supports optional communication via TLS, this is particularly useful when the database is not located on the same host, such as when transferring data over the internet.

Implementation details can be found on https://mariadb.com/kb/en/library/1-connecting-connecting/#client-handshake-response. The implementation very likely depends on STARTTLS support via reactphp/socket#89.