Problem: memiavl leaf node separation optimization is not implemented

[yihuang]

Closes: #1016

20% improvement in RandomGet benchmark.

$ go test -run=^$ -bench="RandomGet/memiavl-disk" -benchmem ./ -count=4 -tags nativebyteorder

$ benchstat /tmp/before.txt /tmp/after.txt
goos: darwin
goarch: amd64
pkg: github.com/crypto-org-chain/cronos/memiavl
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
                          │ /tmp/before.txt │           /tmp/after.txt            │
                          │     sec/op      │    sec/op     vs base               │
RandomGet/memiavl-disk-12      175.8n ± ∞ ¹   138.2n ± ∞ ¹  -21.37% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

                          │ /tmp/before.txt │         /tmp/after.txt         │
                          │      B/op       │    B/op      vs base           │
RandomGet/memiavl-disk-12       0.000 ± ∞ ¹   0.000 ± ∞ ¹  ~ (p=1.000 n=4) ²
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

                          │ /tmp/before.txt │         /tmp/after.txt         │
                          │    allocs/op    │  allocs/op   vs base           │
RandomGet/memiavl-disk-12       0.000 ± ∞ ¹   0.000 ± ∞ ¹  ~ (p=1.000 n=4) ²
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

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• Have you read the CONTRIBUTING.md?
• Does your PR follow the C4 patch requirements?
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• Have you included tests for any non-trivial functionality?
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Thank you for your code, it's appreciated! :)

[github-advanced-security]

gosec found more than 10 potential problems in the proposed changes. Check the Files changed tab for more details.

[codecov]

Codecov Report

Merging #1021 (088108a) into main (6765a82) will increase coverage by 0.38%.
The diff coverage is 75.41%.

@@            Coverage Diff             @@
##             main    #1021      +/-   ##
==========================================
+ Coverage   45.32%   45.71%   +0.38%     
==========================================
  Files          80       80              
  Lines        6718     6895     +177     
==========================================
+ Hits         3045     3152     +107     
- Misses       3354     3411      +57     
- Partials      319      332      +13     

Impacted Files	Coverage Δ
memiavl/snapshot.go	54.64% <60.95%> (-1.22%)	⬇️
memiavl/import.go	64.44% <66.66%> (-0.20%)	⬇️
memiavl/export.go	65.42% <67.69%> (-8.27%)	⬇️
memiavl/persisted_node.go	95.20% <94.11%> (-4.80%)	⬇️
memiavl/iterator.go	79.71% <100.00%> (ø)
memiavl/layout_little_endian.go	100.00% <100.00%> (ø)
memiavl/mem_node.go	95.08% <100.00%> (ø)
memiavl/mmap.go	68.75% <100.00%> (+8.75%)	⬆️
memiavl/node.go	73.43% <100.00%> (-0.41%)	⬇️