remove called unsubscribe callbacks from list of callbacks

[sjhewitt]

When unsubscribing from pubsub channels, a reference to expired callbacks was kept.

This change updates the unsubscribe_callbacks list to remove references once a callback has been called

[AeroNotix]

Any update on this? I'm seeing leaking clients when unsubscribing.

[AeroNotix]

@sjhewitt If you push/amend the commit, travis will try again, looks like the failures weren't related to your changes.

[AeroNotix]

Tested this locally, does not fix a client leak when unsubscribing. The websocket example leaks clients. @sjhewitt @leporo any ideas?

[sjhewitt]

Can you create a failing test case or bit of example code?