Delay clearing of state in slaves

until sender down notification is received from channel as well as GM
in order to avoid messages being enqueued more than once
Rename and remove duplicate credit handing on peer down
Revert b2db7048b839 (which in turn mostly reverted dd08c9204760), i.e. return to the broken bug25644 implementation.
Store monitors correctly, with a two level dictionary, mapping MonitoredPid -> MonitoringPid -> Ref and MonitoredPid -> Ref -> MonitoringPid.
Explain what this is.
Make delete and unbind idempotent.
Simpler to maintain MonitoredPid -> {Ref, MonitoringPidSet}.
ahem
we only support process monitoring, so enforce that in the API
we don't really support demonitor/2, so rip it out
due to the aggregration of all monitors of a single pid into a single
monitor, the semantics of demonitor/2 is decidely dodgy
cosmetics, better var names, and optimise 'DOWN' sending
Use gb_sets rather than sets for performance.
refactor: make proper use of gb_sets API
merge bug25714 into default
Merge bug25602
banish side effect dependent code
merge bug25721 into stable
merge stable into default
Merged bug25716 into default
Merge default