Don't submit unnecessary keep-alives

[kuujo]

Currently, clients submit keep-alive requests on a fixed schedule to ensure their session isn't expired. However, for clusters that are fairly active, these keep-alives represent unnecessary overhead as they require a commit within the Copycat cluster. When commands are being submitted within the session timeout, the commands can serve the same purpose as keep-alives. Thus, clients should track timers for commands submitted to the cluster and only submit keep-alives when necessary to reset indexes or to keep the session alive.