Shift multi-callback management from OperationalDeviceProxy to CASESessionManager

[mrjerryjohns]

Problem

Comments like this one reveal that clean-up of OperationalDeviceProxy instances on failure is just fraught with peril. Currently, callers of FindOrEstablishSession are responsible for releasing OperationalDeviceProxy instances on error. This is really error-prone. The ability for OperationalDeviceProxy to support multiple registered callbacks on the same instance complicates this story.

Proposal

Extending the idea in that comment, up-level the multi-callback management scheme in OperationalDeviceProxy to CASESessionManager. This simplifies OperationalDeviceProxy down to just managing a single callback, while permitting CASESessionManager to now take-over the responsibility of free'ing up the proxy instance on error, while still retaining the multi-callback capability of the high-level FindOrEstablishSession API.

[mrjerryjohns]

FYI @msandstedt @bzbarsky-apple

[bzbarsky-apple]

So the CASESessionManager would need to keep track of which callbacks go with which proxy, right? But yes, we could do that...

[msandstedt]

We use the SecureSessionTable and Sessions directly, so aren't exposed to any of this. I defer to others on how OperationalDeviceProxy and CASESessionManager should work together.

[mrjerryjohns]

Dupe of #19259