Track communication resource leaks

[ecpeterson]

The communication manager is not especially good at tracking lost / leaked objects. Primarily, these include unanswered (and unanswerable) messages, as well as channels that are allocated and discarded without being released. Find a way to make these objects safer and to warn when they're misused.

The HASH-TABLEs held by a given COURIER might be converted to weak hash tables as supplied by TRIVIAL-GARBAGE. This would probably have the effect that unintended memory leaks are quietly covered up. Since detecting such leaks will be important for hardware implementations, covering them up is a worse option.