Backpressure mitigation option

The current mode for handling too much backpressure (maxBackpressure) is to skip sending more. This works fine for application protocols that have no hard dependency on history, but silently breaks those who do.

• closeOnBackpressureLimit is a new option that takes bool. The default will be false (skip sending without closing).
• To avoid classic reentry bugs, closeOnBackpressureLimit = true will defer the close handler until next event loop iteration. It will not emit immediately.