* Tolerate shutdown message after channel is closed
### Motivation
A channel can close unexpectedly if something goes wrong. We may in the meantime have scheduled the connection for graceful shutdown but the connection has not yet seen the message. We need to still accept the shutdown message and just ignore it if we are already closed.
### Modification
- ignore calls to shutdown if the channel is already closed
- add a test which would previously crash because we have transition from the closed state to the closing state and we hit the deinit precondition
- include the current state in preconditions if we are in the wrong state
### Result
We don’t hit the precondition in the deinit in the scenario described above and have more descriptive crashes if something still goes wrong.
### Motivation
Our current swiftformat version does not support async/await. Since we want to add support for async/await we must update swiftformat or disable it. I tried my very best to keep the number of changes as small as possible. I assume we want to stick with the new 0.48.8 for some time.
### Changes
- Update swiftformat to 0.48.8
### Result
We can land async/await code.
David Nadoba
Fabian Fett