Problem: memiavl more efficient snapshot setup is not used

[yihuang]

Solution:

• only export state-sync snapshot on a snapshot, which is more efficient than export an arbitrary version

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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! :)

[codecov]

Codecov Report

Merging #1067 (eafec05) into main (db62342) will increase coverage by 23.34%.
The diff coverage is 35.84%.

@@             Coverage Diff             @@
##             main    #1067       +/-   ##
===========================================
+ Coverage   22.74%   46.09%   +23.34%     
===========================================
  Files          50       82       +32     
  Lines        3025     7127     +4102     
===========================================
+ Hits          688     3285     +2597     
- Misses       2272     3492     +1220     
- Partials       65      350      +285     

Impacted Files	Coverage Δ
app/memiavl.go	8.69% <0.00%> (-91.31%)	⬇️
memiavl/export.go	58.75% <34.78%> (ø)
memiavl/db.go	57.72% <68.75%> (ø)

... and 46 files with indirect coverage changes