core: reuse buffers in persist()

[fyrchik]

When flushing many key-value pairs, cost of a buffer grow (reallocation) can become significant.
This PR reduces it by reusing the same buffer for (*cacheddao).Persist().

[codecov]

Codecov Report

Merging #772 into master will increase coverage by 0.01%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   66.94%   66.96%   +0.01%     
==========================================
  Files         141      141              
  Lines       13034    13045      +11     
==========================================
+ Hits         8726     8735       +9     
- Misses       3898     3900       +2     
  Partials      410      410

Impacted Files	Coverage Δ
pkg/core/cacheddao.go	83.9% <100%> (+0.77%)	⬆️
pkg/core/dao.go	69.87% <90.9%> (+0.03%)	⬆️

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[roman-khimov]

I think I'm OK with fd6d3c1 if you're to change the API (dao and cachedDao are supposed to be interchangeable), but I don't think 0eb4b4f makes much sense, it can be dangerous, it complicates code, but the gain is probably not that huge (1 new buffer per 10-100 serializations is not a lot of overhead).