Oom fixes 2022.01.19

[iulianpascalau]

• made the db pointer inside the storers implementation nil whenever we call Close or Destroy. Added nil checks on all usages of the db pointer.
• peer accounts trie snapshot for shard nodes so the pruning storer for the peer accounts will not cause OOMs
• minor storers refactor: used context instead of chan

[codecov]

Codecov Report

Merging #3709 (5c7f6c7) into development (9e8e91a) will increase coverage by 0.00%.
The diff coverage is 85.62%.

@@             Coverage Diff              @@
##           development    #3709   +/-   ##
============================================
  Coverage        73.78%   73.79%           
============================================
  Files              589      589           
  Lines            76218    76312   +94     
============================================
+ Hits             56241    56316   +75     
- Misses           15530    15545   +15     
- Partials          4447     4451    +4     

Impacted Files	Coverage Δ
process/block/shardblock.go	67.07% <0.00%> (-0.66%)	⬇️
storage/leveldb/serialActions.go	73.52% <66.66%> (-11.48%)	⬇️
storage/leveldb/leveldbSerial.go	81.86% <92.30%> (+1.97%)	⬆️
storage/pruning/pruningStorer.go	72.90% <93.10%> (+1.02%)	⬆️
storage/leveldb/leveldb.go	81.56% <94.73%> (+0.10%)	⬆️
factory/blockProcessorCreator.go	79.52% <100.00%> (+0.02%)	⬆️
state/peerAccountsDB.go	87.64% <100.00%> (+2.11%)	⬆️
storage/leveldb/common.go	83.33% <100.00%> (+4.76%)	⬆️
storage/pruning/fullHistoryPruningStorer.go	77.77% <100.00%> (+2.33%)	⬆️
storage/pruning/triePruningStorer.go	66.66% <100.00%> (ø)
... and 5 more

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[gabi-vuls]

System test passed.