Recovering lost parked nodes

[poszu]

Recover parked nodes and the top layer of the tree from the layercache.bin files saved on disk instead from the round's state.bin.

Parked nodes recovery

Parked nodes on disk-cached layers are recovered from the files (representing the tree's bottom layers). If a layer contains an odd number of nodes/leafs, the last one is a parked node.

Parked nodes on mem-cached layers are recovered by rebuilding a subtree, in which the bottom layer is built from the nodes in the top disk-cached layer.

mem-cached nodes recovery

The top of the tree that is not persisted on disk is rebuilt as the aforementioned subtree. It's layers from 1 to the root are moved into the recovered tree.

TODO:

• remove parked nodes completely from the round execution state and rely solely on recovery implemented in this PR.

[noamnelke]

[codecov]

Codecov Report

Merging #344 (3123490) into develop (ec4b107) will decrease coverage by 0.3%.
The diff coverage is 64.6%.

@@            Coverage Diff            @@
##           develop    #344     +/-   ##
=========================================
- Coverage     73.1%   72.8%   -0.3%     
=========================================
  Files           17      17             
  Lines         1568    1641     +73     
=========================================
+ Hits          1147    1196     +49     
- Misses         318     334     +16     
- Partials       103     111      +8     

Files Changed	Coverage Δ
prover/prover.go	60.9% <63.8%> (+1.3%)	⬆️
service/round.go	77.6% <70.0%> (-0.8%)	⬇️

... and 1 file with indirect coverage changes