Solve memory leak caused by `Task.WhenAny` usage

[nordfjord]

Fixes leak per cycle in:

• the Sink's ingester (i.e. a leak per batch submitted, including empty ones at tail)
• The Scheduler's main loop (one per dispatched set of work, which typically is min 60 iterations per minute)

Before (5.83MB in Gen2 at 1m):

After (3MB in Gen2 at 1m and less memory pressure over all):

See the test code I ran to get these profiles here

Task.WhenAny doesn't cancel the rest of the Tasks when one succeeds, therefore we must create a linked cancellation token to cancel the stuff we don't want to leak.

[bartelink]

Great work, thank you! If only I'd have managed to infer all Stephen Cleary was implying in https://stackoverflow.com/questions/73233208/using-task-whenany-to-await-capacity-on-a-semaphoreslim/73233209#73233209 at the time and been able to get it right in the first instance... Ping me when it's ready to merge and I'll get it out the door ASAP (will also ship https://github.com/jet/equinox/pull/394 at the same time as there are (not) static types I need to Sealed; AbstractClass)