TuningAgent interface

[wangpatrick57]

Summary: basic TuningAgent interface. It provides a common interface between agents and the replay subsystem (which will be pushed in a later PR).

Demo:
Added test cases that pass.

Details:

• The core of the interface is the DBMSConfigDelta class which represents the change to the DBMS's config after a single step of tuning.
• Subclasses only need to override the TuningAgent._step() function.
• The main functionality of the class is in automatically saving the deltas to files.
• CWI's JOB dataset link also stopped working so I made my own: https://drive.google.com/uc?id=19m0zDpphAw0Bu9Irr_ta9EGr5k85hiN1.