feat: sched: Add scheduler interfaces, configurable assigner

[magik6k]

Related Issues

Proposed Changes

• Add some more interfaces to the scheduling logic, which should make it easier to customize scheduling logic
• Add config allowing selection of worker assigners.
• Introduce new, simpler worker assigning logic - when enabled, the "spread" assigner will ignore worker utilization, and will attempt to assign tasks to as many workers as possible

To use the new assigner:

[Storage]
 # Assigner specifies the worker assigner to use when scheduling tasks.
 # "utilization" (default) - assign tasks to workers with lowest utilization.
 # "spread" - assign tasks to as many distinct workers as possible.
 #
 # type: string
 # env var: LOTUS_STORAGE_ASSIGNER
 Assigner = "spread"

TODO

• Take worker load in the spread assigner into account (don't assign too much)
• Test the spread assigner at least somewhat

[codecov]

Codecov Report

Merging #8700 (3de34ea) into master (df98a2a) will increase coverage by 0.05%.
The diff coverage is 89.11%.

@@            Coverage Diff             @@
##           master    #8700      +/-   ##
==========================================
+ Coverage   40.82%   40.88%   +0.05%     
==========================================
  Files         688      691       +3     
  Lines       75916    75992      +76     
==========================================
+ Hits        30996    31068      +72     
+ Misses      39579    39563      -16     
- Partials     5341     5361      +20     

Impacted Files	Coverage Δ
extern/sector-storage/manager.go	61.95% <50.00%> (-0.15%)	⬇️
extern/sector-storage/selector_task.go	61.11% <66.66%> (+27.77%)	⬆️
extern/sector-storage/sched_assigner_common.go	78.75% <78.75%> (ø)
extern/sector-storage/request_queue.go	86.66% <81.81%> (ø)
extern/sector-storage/sched.go	82.89% <82.22%> (-3.04%)	⬇️
extern/sector-storage/sched_assigner_spread.go	91.48% <91.48%> (ø)
extern/sector-storage/sched_post.go	81.60% <92.30%> (ø)
extern/sector-storage/sched_worker.go	79.41% <95.45%> (+0.58%)	⬆️
...xtern/sector-storage/sched_assigner_utilization.go	100.00% <100.00%> (ø)
extern/sector-storage/sched_resources.go	86.45% <100.00%> (ø)
... and 28 more