sealing: Improve scheduling of ready work

[magik6k]

Right now if we have a task which is preparing, another task which is behind it that is ready, and there is nothing currently executing we won't start the ready task even though we could. This PR makes that logic smarter.

Might even work right now; but still some things remain todo:

• Make sure that ready-scheduled tasks are executed in correct order (I think this is fine??)
• Ready checks for more task types
  • Can be a follow up PR
• Run on a miner for a while
• Run on some more miner setups

On top of #7334

[codecov]

Codecov Report

Merging #7335 (9af82f2) into master (fc10281) will increase coverage by 0.02%.
The diff coverage is 77.46%.

@@            Coverage Diff             @@
##           master    #7335      +/-   ##
==========================================
+ Coverage   39.54%   39.56%   +0.02%     
==========================================
  Files         616      616              
  Lines       65295    65360      +65     
==========================================
+ Hits        25818    25858      +40     
- Misses      34980    35012      +32     
+ Partials     4497     4490       -7     

Impacted Files	Coverage Δ
extern/sector-storage/sched.go	84.71% <ø> (ø)
extern/sector-storage/sched_worker.go	77.84% <75.38%> (-1.28%)	⬇️
extern/sector-storage/sched_resources.go	85.29% <100.00%> (+0.91%)	⬆️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
miner/miner.go	52.31% <0.00%> (-3.32%)	⬇️
extern/sector-storage/stores/local.go	56.81% <0.00%> (-1.99%)	⬇️
chain/exchange/client.go	52.96% <0.00%> (-1.70%)	⬇️
itests/kit/blockminer.go	93.65% <0.00%> (-1.59%)	⬇️
... and 18 more

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