dispatch queue back-pressure and deadlock

dispatch queue can deadlock:

* if a pubsub connection both publishes and subscribes to the same channel and sends multiple messages that the queue is:
` Publish <----- Publish <----- Publish <-----` ends up `Publish <----- SendPubMessage <----- SendPubMessage <----- SendPubMessage` and deadlocks because the top message `Publish` now will block on `EnsureBelowLimit`.

* If intrusive messages populate the queue recursively nonstop then the rest of the queue will starve

The issue names no files, tests, or entry points. Begin by reproducing the pubsub scenario where Publish and SendPubMessage deadlock under queue back-pressure, then investigate the recursive intrusive-message case; done means neither deadlocks nor starves the rest of the dispatch queue.

Written by the indexing model from the issue text.