Ensures rolling upgrades from 1.9.5 to 1.9.6

Reintroduces snapshot requests from joining instances. The reason is that
 1.9.5 instances do not provide any means of discovering the state of the
 cluster other than snapshots. So if we want 1.9.6 instances to properly
 join they have to ask and receive snapshots. But, we do not want 1.9.6
 instances to actually serve snapshots, so now the coordinator returns
 a null snapshot state which is detected and discarded. This should
 allow the 1.9.6 instances to work problem free while being able
 to replace a 1.9.5 cluster.