Matthias' strawman patch: prioritise notify_sent based on current ingress / egress rates.
Add a BQ function for this.
Prioritise everything we used to before bug 24966.
Don't prioritise 'reject', it could lead to observable reordering.
Merge in default
Explicitly target an egress/ingress ratio.
Update rates every 1000 messages or at the ram duration timeout, whichever comes first.
Unify the #rates record so we don't duplicate as much stuff.
Cosmetic
oops
Rates on exponential moving average.
Try to get the weighting factor right.
The existing value can be 'undefined'.
Introduce a new parallel (multi-server) call API
Refactor: tag replies safely and avoid using erlang:demonitor/2

For efficiency reasons, instead of demonitoring (which leads to scanning the
entire mailbox), cope with duplicate monitor refs when collating gen_server
replies.

For safety, tag the exit signal (which we treat as a result) with a unique
reference instead of the middle-man's pid.
Handle varying destinations in mcall/1
Add a note to the gs2 essay
Use gs2:mcall/1 in r_amqqueue:force_event_refresh/1
Fix multi-call tests
Remove rabbit_misc:multi_call/2