Enable pool_pre_ping in DB connection
Enable pool_pre_ping in DB connection

To avoid problems with inactive DB connections, SQLAlchemy provides a `pool_pre_ping` option, that described in more detail here:

http://docs.sqlalchemy.org/en/latest/core/pooling.html#disconnect-handling-pessimistic

In flask environments, it's enabled by subclassing SQLAlchemy, which is what I've done here.

Fixes errors like:
sqlalchemy.exc.OperationalError: (_mysql_exceptions.OperationalError) (2006, 'MySQL server has gone away') which results in an Error 500 in the UI.

ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin