Fix for case where db_pool connections can be lost.

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.

eventlet/db_pool.py

@@ -180,7 +180,7 @@ def get(self):
         if conn is None:
             try:
                 conn = self.create()
-            except Exception:
+            except:
                 # unconditionally increase the free pool because
                 # even if there are waiters, doing a full put
                 # would incur a greenlib switch and thus lose the