Refactor Trie hasher using onRoot flag

[blukat29]

Proposed changes

De-duplicate code in the trie hasher (storage/statedb/hasher.go).

• Current implementation has two pairs of similar functions:
  • single-threaded version hash() + hashChildren()
  • multi-threaded version hashRoot() + hashChildrenFromRoot()
• This PR removes the latter. Instead, the onRoot argument switches multi-threading.
  • hashNode(..., onRoot) + hashChildren(..., onRoot) runs multi-threaded if onRoot == true.
    • hash(...) = hashNode(..., onRoot=false)
    • hashRoot(...) = hashNode(..., onRoot=true)

Types of changes

• Bugfix
• New feature or enhancement
• Others

Checklist

• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

Further comments

Benchmark results show that this PR does not affect hashing performance (on Apple M1 Max 10-core)

• Before PR

   $ go test -run=^$ -bench=BenchmarkHash
   BenchmarkHash-10                 2582688               402.2 ns/op           684 B/op          7 allocs/op
   BenchmarkHashFixedSize/10-10       26340             43323 ns/op           14891 B/op        179 allocs/op
   BenchmarkHashFixedSize/100-10              12986             89688 ns/op           66881 B/op        809 allocs/op
   BenchmarkHashFixedSize/1K-10                2024            618712 ns/op          659172 B/op       7811 allocs/op
   BenchmarkHashFixedSize/10K-10                282           4171724 ns/op         6907710 B/op      79703 allocs/op
   BenchmarkHashFixedSize/100K-10                33          35102691 ns/op        68240330 B/op     794537 allocs/op
  

• After PR

   BenchmarkHash-10                 3081148               439.9 ns/op           677 B/op          7 allocs/op
   BenchmarkHashFixedSize/10-10       27524             42308 ns/op           14262 B/op        178 allocs/op
   BenchmarkHashFixedSize/100-10              13624             88954 ns/op           66293 B/op        808 allocs/op
   BenchmarkHashFixedSize/1K-10                2050            604276 ns/op          658532 B/op       7810 allocs/op
   BenchmarkHashFixedSize/10K-10                280           4168147 ns/op         6906603 B/op      79701 allocs/op
   BenchmarkHashFixedSize/100K-10                30          38524168 ns/op        68240050 B/op     794539 allocs/op
  

• (for comparison) After PR, if onRoot == false

   BenchmarkHash-10                  564274              2035 ns/op             628 B/op          7 allocs/op
   BenchmarkHashFixedSize/10-10       41187             28873 ns/op           13213 B/op        155 allocs/op
   BenchmarkHashFixedSize/100-10               8718            134050 ns/op           64321 B/op        772 allocs/op
   BenchmarkHashFixedSize/1K-10                 842           1415162 ns/op          649540 B/op       7742 allocs/op
   BenchmarkHashFixedSize/10K-10                 70          17122263 ns/op         6865380 B/op      79500 allocs/op
   BenchmarkHashFixedSize/100K-10                 7         164345345 ns/op        68196154 B/op     794339 allocs/op	
  

[ethan-kr]

I made a comment, but there is no disagreement, so I approve it.

[jeongkyun-oh]

LGTM

[blukat29]

@ethan-kr PTAL :)