Call Swift connection close callback from global executor

[externl]

This PR changes the behavior of the Connection close callback to call back from a Swift global executor thread and not a C++ thread pool thread. It also updates the callback to be async.

I think this is the last place where C++ threads could call into Swift user code.

[bernardnormier]

I prefer to keep the synchronous callback. From the C++ point of view, this callback is a synchronous callback that should not block, and it's fine for the Swift code to use the C++ thread. If the Swift code needs to call an async function as part of this callback, it can/should create a Task.