Timing of SQL queries fixed by using the new Events API from SQLAlchemy.

Timing of SQL queries fixed by switching to the new Event API from SQLAlchemy introduced in version 0.7. Usage of ConnectionProxy is deprecated from that version, and using it resulted in invalid measures.

This fix requires SQLAlchemy >= 0.7, but since the code of flask-sqlalchemy is already using the Events API from SQLAlchemy, there would be no change in that regard.