Alter ENGINE to mysql_pool. Add some DATABASE_POOL_ARGS for example:

DATABASE_POOL_ARGS = {
    'max_overflow': 10,
    'pool_size':5,
    'recycle':300
}

Optionally add a sa_pool_key value so we don't need to hash each db.

Example:

'two': {
    'ENGINE': 'mysql_pool',
    'sa_pool_key': 'two',
    ...
}

Require SQL Alchemy 0.7 for the events.