Separate app store

[rus-alex]

App's data is stored in a dedicated DB from now.
This change is a step to fully decoupled app and gossip logic.

[codecov-io]

Codecov Report

Merging #547 into develop2 will decrease coverage by 0.37%.
The diff coverage is 22.11%.

@@             Coverage Diff              @@
##           develop2     #547      +/-   ##
============================================
- Coverage     43.62%   43.25%   -0.38%     
============================================
  Files           162      162              
  Lines          9984    10065      +81     
============================================
- Hits           4356     4354       -2     
- Misses         5214     5296      +82     
- Partials        414      415       +1     

Impacted Files	Coverage Δ
app/apply_genesis.go	0.00% <0.00%> (ø)
app/store_epoch_stats.go	0.00% <0.00%> (ø)
app/store_migration.go	0.00% <0.00%> (ø)
app/store_sfc_delegators.go	0.00% <ø> (ø)
cmd/lachesis/export.go	0.00% <0.00%> (ø)
evmcore/dummy_block.go	6.25% <0.00%> (-1.45%)	⬇️
gossip/config.go	100.00% <ø> (ø)
gossip/ethapi_backend.go	0.31% <0.00%> (ø)
gossip/store_refund.go	65.38% <ø> (ø)
gossip/store_migration.go	5.19% <2.73%> (+5.19%)	⬆️
... and 19 more

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