Implementing Storage.Add on verification mempool cache

[igormcoelho]

Draft implementation for: #814

[igormcoelho]

Just to explain what I'm trying to do... right now, to make it easier, I created two interops: Storage.PutCache and Storage.GetCache. These will first access mempool cache (if Trigger is Verification), and then the persisted value, if not found on cache.
On the future, these can perhaps be merged (or not) with Storage.Get and Storage.Put (or PutEx) for more transparent usage... I don't know yet.

[igormcoelho]

As advised by @ricklock9 (#814 (comment)) perhaps a better interface is something like Storage.IncreaseBy and Storage.DecreaseBy, since it allows only basic additive/subtractive stuff. Or more simply Storage.Add. Thanks Ricardo!

[codecov-io]

Codecov Report

Merging #820 into master will decrease coverage by 0.05%.
The diff coverage is 5%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   38.41%   38.36%   -0.06%     
==========================================
  Files         176      176              
  Lines       12459    12479      +20     
==========================================
+ Hits         4786     4787       +1     
- Misses       7673     7692      +19

Impacted Files	Coverage Δ
neo/Persistence/Snapshot.cs	78.94% <ø> (ø)	⬆️
neo/SmartContract/InteropService.cs	20.39% <5%> (-0.7%)	⬇️

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[codecov-io]

Codecov Report

Merging #820 into master will decrease coverage by <.01%.
The diff coverage is 41.02%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   38.41%   38.41%   -0.01%     
==========================================
  Files         176      176              
  Lines       12459    12496      +37     
==========================================
+ Hits         4786     4800      +14     
- Misses       7673     7696      +23

Impacted Files	Coverage Δ
neo/SmartContract/Native/Tokens/Nep5Token.cs	96.4% <100%> (ø)	⬆️
neo/SmartContract/InteropService.cs	20.39% <5%> (-0.7%)	⬇️
neo/Persistence/Snapshot.cs	77.77% <76.47%> (-1.17%)	⬇️

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[igormcoelho]

@shargon @erikzhang this is the core of the idea... not complete yet, but given an overview of the changes needed on Nep5Token class. Can you take a look please, just to evaluate the direction?
I know it's not beautiful the code hahaha please, just the direction... ;)

[igormcoelho]

In fact, to implement this, we would first need to detach balance from TState objects: #831
Issue: #830

[igormcoelho]

Not feasible anymore... however, lead to nice improvements, such as garbage collection and dettached TState objects ;)