Implement scheduler

[renecannao]

WHY

ProxySQL is becoming a complex system. Its core should stay as simple as possible.
Therefore, complex logic should be moved outside the core, and called asynchronously.
This will allow faster development, as every user can implement their own set of scripts and share them.

WHAT

• scheduler table
• commands to load and save scheduler table
• commands to load the scheduler from config file
• call the scheduler from within the Admin main loop
• document it

[renecannao]

Implementation started in bda2cf4