Implement MySQL Monitor module

[renecannao]

ProxySQL needs to constantly monitor its backends to determine their status.
This module will be responsible to only monitor the backends: won't perform any corrective action.

[aismail]

@renecannao if you want to keep track of multiple issues under a parent issue, I would advise you to make a list of them in the parent issue (and link to them from there). Much easier to track.

[renecannao]

Child issues:

• MySQL Monitor module should access global variables via MySQL_Thread #280 : MySQL Monitor module should access global variables via MySQL_Thread
• Add global variables for MySQL Monitor module #282 : Add global variables for MySQL Monitor module
• Clean up MySQL_Thread with regards to backend monitoring #283 : Clean up MySQL_Thread with regards to backend monitoring
• Use MariaDB Client Library to connect to backend from MySQL Monitor Module #284 : Use MariaDB Client Library to connect to backend from MySQL Monitor Module