vm,dao: return storage iterator from DAO in Storage.Find interop

[fyrchik]

Fix #988 .

Reproduce behavior of the reference realization:

• if item was Put in cache after it was encountered during
  Storage.Find, it must appear twice
• checking if item is in cache must be performed in real-time
  during Iterator.Next()

Need too retest #822 .

[fyrchik]

Linter is unhappy, because we return unexported type. But it is intended to be opaque. Maybe we should just return an interop item.

[roman-khimov]

Or maybe we can move storageWrapper out of vm? I don't think it should be a part of vm.

[codecov]

Codecov Report

Merging #992 into master-2.x will decrease coverage by 0.12%.
The diff coverage is 38.33%.

@@              Coverage Diff               @@
##           master-2.x     #992      +/-   ##
==============================================
- Coverage       67.97%   67.85%   -0.13%     
==============================================
  Files             145      146       +1     
  Lines           14320    14359      +39     
==============================================
+ Hits             9734     9743       +9     
- Misses           4126     4155      +29     
- Partials          460      461       +1     

Impacted Files	Coverage Δ
pkg/core/blockchain.go	44.81% <0.00%> (ø)
pkg/core/dao/cacheddao.go	37.00% <0.00%> (-5.21%)	⬇️
pkg/core/dao/dao.go	49.84% <0.00%> (ø)
pkg/core/interop_system.go	36.57% <0.00%> (ø)
pkg/vm/interop_iterators.go	95.23% <ø> (ø)
pkg/core/interop_neo.go	57.27% <75.00%> (-0.48%)	⬇️
pkg/core/storage_find.go	88.23% <88.23%> (ø)
pkg/vm/interop.go	90.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 79c87ca...b0d07c3. Read the comment docs.

[roman-khimov]

pkg/vm/kek?

[roman-khimov]

Confirming that it actually fixes #977 too:

2020-05-27T11:30:15.241+0300    INFO    runtime log     {"script": "acdbf5346b59185b0e0df88fead2a7e856df1ab5", "logs": "\"Transfer is failed. fromBalance < value\""}
2020-05-27T11:30:15.241+0300    WARN    contract invocation failed      {"tx": "61366ab9fe37e100854446ab30005d7afbb093096670592ed8d499e3b6a6f753", "block": 2400907, "error": "error encountered at instruction 79 (THROWIFNOT): THROWIFNOT"}

Quick application log dump:

&{53f7a6b6e399d4d82e5970660993b0fb7a5d0030ab46448500e137feb96a3661 Application FAULT 5.645 [] []}