CO connection/heap coelescing

[eswdd]

We want to add the capability such that a client can choose to migrate a subscription to a particular heap uri to a different server. This situation might occur if a request to subscribe comes in, the uri returned by the server matches one that another server has already returned, and so the client would prefer to receive and process only one stream of updates rather than many copies of the same thing.

We would probably want to do this as part of the subscription process, and should be toggleable.

It's also likely that the client would want to be able to direct the initial subscription request to the same server where the parameters (or a certain subset of parameters) for a particular CO method match previous requests - essentially some kind of subscription request to server affinity.