SqlAlchemy mysql connections drop and crash the session

[henryhbk]

The classic "the MySQL server has gone away" error. This is due to Flask timing out an idle connection pool in the sqlalchemy engine and is fixed by either the pessimistic or optimistic strategies specified in the engine docs:

[henryhbk]

Fixed with commit #90978fd adding pool_pre_ping=True per SqlAclhemy docs (Pessimistic strategy)