fix #2974 -- use redis::aio::ConnectionManager

[fibonacci1729]

Fixes #2974

This replaces the use of redis::aio::MultiplexedConnection with redis::aio::ConnectionManager which automatically attempts to retry connecting upon error. See this for behavior of ConnectionManager. To handle not passing on the first error to the user as described by "When a command sent to the server fails with an error that represents a “connection dropped” condition, that error will be passed on to the user..." relevant implementations of Store now implement after_open() which will to try ping the connection to trigger the automatic retry.

[dicej]

LGTM. It's a little weird that we have to clone the ConnectionManager to call any redis::Commands methods, but I guess it makes more sense than adding a layer of Arc<Mutex<...>> on top of it.

[itowlson]

did I mention