MySQL proxy improvements (#289)

This PR further improves and refactors the original MySQL proxy
prototype. Notably:
- Add relevant docs and manual links to all packet manipulation methods.
- Refactor packet building, fix packet sequence ID handling.
- Refactor packet parsing and authentication.
- Use `pack()` and `unpack()` when possible.
- Fix connecting with no password specified.
- Support specifying database name during the authentication phase
(support the `MySQL_Protocol::CLIENT_CONNECT_WITH_DB` capability).
- Correctly support the `MySQL_Protocol::CLIENT_DEPRECATE_EOF`
capability.
- Add MySQL CLI tests.

I've tried the proxy with the following tools, which are working now:
- MySQL CLI
- Adminer
- phpMyAdmin
- TablePlus

I think this is a good point to stop at for this PR. The refactor may go
further, and there are more things to implement (prepared statements for
`mysqli`, full authentication support, and more), but I'd leave those to
separate issues.

I'm also considering moving the packet building methods from
`MySQL_Protocol` to `MySQL_Session`, as currently, these need to pass
many function parameters in some cases, while in the `MySQL_Session`
class, private state properties could be used directly for some
attributes.

Author: adamziel