The problem occurs if:
(1) A greenthread was waiting for a connection from the pool
(2) Another thread calls put() to return a connection to the pool,
    but it has expired, so the first thread will be cleared to
    create a new connection
(3) The create() method raises an exception that doesn't inherit
    from Exception, such as eventlet.Timeout

Note that eventlet.pools.Pool.get uses "except:" instead of
"except: Exception" to make sure all exceptions from create() are
accounted for, but eventlet.db_pool.get() incorrectly uses
"except: Exception" in the case where a returned connection was
found to be unusable.