store: change PruningDefault to KeepEvery:100

[erikgrinaker]

Change the default pruning strategy to KeepEvery:100, the same as 0.38.4. This is to reduce the chance of data loss for users blindly upgrading from 0.38.0-0.38.4, due to bugs in the IAVL pruning implementation. See 0.38.5 change log for details.

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[codecov]

Codecov Report

Merging #6520 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6520   +/-   ##
=======================================
  Coverage   56.36%   56.36%           
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  Files         470      470           
  Lines       28234    28234           
=======================================
  Hits        15913    15913           
  Misses      11195    11195           
  Partials     1126     1126