Implement splitstore cold object reification

[vyzo]

See #6726 for context.

This implements a specialized view for the splitstore which reifies cold objects on hotstore misses.

The view is hooked to the VM through the constructor for now but I am not sure that's the right place to do it.

One alternative would be to change Chainstore.StateBlockstore() to take a hotview parameter, but that would touch too many call sites.
But maybe that's the right thing to do, open to discussion about this.

Another alternative is to make a Chainstore.StateBlockstoreHotView() and call that when we want to use the hotview.
We'd have to go through the vm constructor call sites and see whether we want to use it.

Edit: the hotview is presented through the StateBlockstore accessor in Chainstore. This ensure that state accesses are reifying. Maybe that's too broad though and we need to introduce and alternate accessor which uses the reifying view, and hunt down callsites to decide what's appropriate.

[vyzo]

rebased on master.

[vyzo]

rebased on master.

[vyzo]

did a rebase and some commit management; there is no need to change the VM, the view should be provided by Chainstore.StateBlockstore.

[vyzo]

rebased on master.

[vyzo]

rebased on master.

[codecov]

Codecov Report

Merging #6914 (ce558b8) into master (d910098) will increase coverage by 0.08%.
The diff coverage is 69.12%.

@@            Coverage Diff             @@
##           master    #6914      +/-   ##
==========================================
+ Coverage   39.84%   39.93%   +0.08%     
==========================================
  Files         632      634       +2     
  Lines       66955    67085     +130     
==========================================
+ Hits        26681    26793     +112     
+ Misses      35683    35668      -15     
- Partials     4591     4624      +33     

Impacted Files	Coverage Δ
blockstore/blockstore.go	62.96% <ø> (ø)
blockstore/splitstore/splitstore_hotview.go	47.61% <47.61%> (ø)
chain/store/store.go	63.34% <50.00%> (+0.34%)	⬆️
blockstore/splitstore/splitstore_reify.go	64.28% <64.28%> (ø)
blockstore/splitstore/splitstore.go	50.56% <92.50%> (+26.69%)	⬆️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
extern/sector-storage/manager_calltracker.go	57.70% <0.00%> (-4.85%)	⬇️
chain/exchange/client.go	52.96% <0.00%> (-4.67%)	⬇️
chain/exchange/peer_tracker.go	66.66% <0.00%> (-4.31%)	⬇️
chain/stmgr/call.go	71.51% <0.00%> (-3.64%)	⬇️
... and 18 more

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[vyzo]

following sync discussion with @magik6k we decided to use a context option for reification and only do it for block validation.

Will rework this after the blockstore upgrade work is done.

[vyzo]

Closing to cherry pick and rework on top of #8008.