Ping redis connection before using it

[retupmoca]

If the redis server closes the connection, we may not know about that closure until we attempt to use it and get a network or aioredis error. If we ping-test the connection first, we can catch and reconnect without causing any application code to receive errors.

[retupmoca]

Should fix #230

[qeternity]

This is no longer required as redis-py pools handle disconnects gracefully.