pw_rpc: ChannelOutput buffer handling fixes

- Ensure ChannelOutput buffers are always released when a call is
  closed.
  * Release any held buffer when aborting a server call due to an error.
  * Permit calling AcquirePayloadBuffer() on an inactive call, but
    return an empty buffer if it is called while inactive. Previously,
    this was disallowed, but it may not be possible to prevent an
    accidental AcquirePayloadBuffer() on a closed call if it is closed
    by a different thread.
  * Release any held buffer when a client call goes out of scope or is
    aborted.
- Extend locking annotations to cover setting callbacks.
- Expose the function to end client streams as CloseClientStream().

Change-Id: I892c66bbeb1701329783b8c360d02ffe25992d98
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/78622
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>