Lots of changes!

Added working get_worker_diff() to DBInterface.py and DB_Mysql_Vardiff.py
Patched out clear_worker_diff() and update_worker_diff() as they are done from the frontend now
Updated basic_share_limiter.py to make use of the DB provided worker diff
Updated basic_share_limiter.py to no longer use variables in config.py for min/max

Progress update:
You can now set VARIABLE_DIFF = True in config.py
When doing so you should set the VDIFF_RETARGET_TIME to the value, in seconds, you wish stratum to check for updates
Ensure your frontend is setting at least difficulty = 1 in pool_worker for each worker

To Do:
Testing...? That's really it!