Updated API DB to use sqlalchemy migrate; enables DB changes without losing data
Added new connection check code for DB (Incase of timeout due to 8 hours of inactivity)
Updates to FlowVisor support code
   - Clean up of code and fixed some potential bugs
   - Improved robustness of code incase of error while deleting FlowSpaces
Updated API DB to support FlowVisor APIs
   - Change necessitated new DB table to store mac2net information
   - Support of mac2net introduces race condition between DB updates and queries, may cause occasional non-fatal exceptions