RPC: optionally close streams on drop.

Per discussion with @Diziet:

We want to have the property where, by default, closing an RPC session will close any RpcDataStream owned by that session. This isn't the default behavior for DataStreamCtrl, so we will need to implement it manually - perhaps by a Drop handler. This should probably be the default behavior: if it isn't, then people are likelier to implement accidental race conditions where their programs work by accident because their RpcDataStream handles are dropped a little later than their connections arrive.

We also need a way to relax this behavior, and allow a DataStream to outlive its RpcDataStream when that is desired. Maybe downgrading to a weak reference is the answer? But it can't be a real weak reference, since nothing else would actually be holding the RpcDataStream.

We also need to decide what happens if an RpcDataStream is released (but not explicitly terminated) by the RPC code.