Transient Replication: Support ring changes when transient replication is in use (add/remove node, change RF, add/remove DC)

The additional state transitions that transient replication introduces require streaming and nodetool cleanup to behave differently. We already have code that does the streaming, but in some cases we shouldn't stream any data and in others when we stream to receive data we have to make sure we stream from a full replica and not a transient replica.

Transitioning from not replicated to transiently replicated means that a node must stay pending until the next incremental repair completes at which point the data for that range is known to be available at full replicas.

Transitioning from transiently replicated to fully replicated requires streaming from a full replica and is identical to how we stream from not replicated to replicated. The transition must be managed so the transient replica is not read from as a full replica until streaming completes. It can be used immediately for a write quorum.

Transitioning from fully replicated to transiently replicated requires cleanup to remove repaired data from the transiently replicated range to reclaim space. It can be used immediately for a write quorum.

Transitioning from transiently replicated to not replicated requires cleanup to be run to remove the formerly transiently replicated data.

nodetool move, removenode, cleanup, decommission, and rebuild need to handle these issues as does bootstrap.

Update web site, documentation, NEWS.txt with a description of the steps for doing common operations. Add/remove DC, Add/remove node(s), replace node, change RF.