Implement @grail/connector-mysql

[sergiodxa]

Implement a connector for the mysql driver.

[XREvo]

I need this connector, i have tried to implement it (commit is on my fork).
I'll test it in the next days. If it can be used, i'll do a PR.
See you soon

[sergiodxa]

@XREvo check the code of the rethinkdb connector, there I used @grial/utils to read the grial.config.js and allow more customization. Also can you check about the usage of a pool instead of a single connection?

[XREvo]

@sergiodxa OK, i'll implement it too