Support relaxed client consistency models

[kuujo]

Clients should be able to optionally relax client-side response/event sequencing to allow clients to progress without awaiting often expensive events. Because some events may be sent from a follower to a client, a client that's connected to a slow follower can be blocked for a relatively long time awaiting an event before the next response can be completed. Clients should be able to optionally weaken guarantees with respect to sequencing of events with responses.