docs: bcachefs: idle work scheduling design doc

People have been asking to see the plan for this, so -

bcachefs has various background tasks that need to be scheduled to
balance efficiency, predict

docs: bcachefs: idle work scheduling design doc

People have been asking to see the plan for this, so -

bcachefs has various background tasks that need to be scheduled to
balance efficiency, predictability of performance, etc.

The design and philosophy hasn't changed too much since bcache, which
was primarily designed for server usage, with sustained load in mind.

These days we're seeing more desktop usage - where we really want to let
the system idle effictively, to reduce total power usage - while also
still balancing previous concerns, we still want to let work accumulate
to a degree.

This lays out all the requirements and starts to sketch out the
algorithm I have in mind.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

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