splistore cold object reification redux

[vyzo]

Implements cold object reification in the splitstore for VM access, as described in #6726; on top of #8008, cherry picked and adapted from #6914.
Closes #6726

[codecov]

Codecov Report

Merging #8029 (6bcade5) into master (049b489) will increase coverage by 0.06%.
The diff coverage is 61.65%.

@@            Coverage Diff             @@
##           master    #8029      +/-   ##
==========================================
+ Coverage   39.20%   39.27%   +0.06%     
==========================================
  Files         662      664       +2     
  Lines       71925    72056     +131     
==========================================
+ Hits        28201    28301     +100     
+ Misses      38839    38837       -2     
- Partials     4885     4918      +33     

Impacted Files	Coverage Δ
blockstore/splitstore/splitstore_reify.go	53.63% <53.63%> (ø)
blockstore/context.go	100.00% <100.00%> (ø)
blockstore/splitstore/splitstore.go	41.47% <100.00%> (+18.50%)	⬆️
chain/consensus/filcns/compute_state.go	74.11% <100.00%> (+0.15%)	⬆️
markets/loggers/loggers.go	88.88% <0.00%> (-11.12%)	⬇️
extern/storage-sealing/sector_state.go	92.30% <0.00%> (-7.70%)	⬇️
chain/events/observer.go	71.64% <0.00%> (-6.72%)	⬇️
markets/retrievaladapter/client_blockstore.go	62.50% <0.00%> (-6.25%)	⬇️
chain/stmgr/execute.go	86.95% <0.00%> (-4.35%)	⬇️
chain/messagepool/repub.go	51.61% <0.00%> (-4.31%)	⬇️
... and 30 more

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[magik6k]

Some mostly questions. Would be good to put answers in code comments as well so that future readers will be less confused.