badger/migration: keep only last version when migrating non-managed DB

[ptrus]

Even though oasis-core always configures non-managed badger databases to keep
a single version of keys, some databases in the wild contain multiple versions
of the same key.

Skip migrating more than the latest version when migrating non-managed
badger databases.

[codecov]

Codecov Report

Merging #4037 (ea119be) into master (d73b8b2) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head ea119be differs from pull request most recent head dfcd789. Consider uploading reports for the commit dfcd789 to get more accurate results

@@            Coverage Diff             @@
##           master    #4037      +/-   ##
==========================================
- Coverage   67.07%   67.06%   -0.01%     
==========================================
  Files         410      410              
  Lines       42148    42151       +3     
==========================================
- Hits        28271    28270       -1     
+ Misses       9894     9892       -2     
- Partials     3983     3989       +6     

Impacted Files	Coverage Δ
go/common/badger/helpers.go	35.29% <0.00%> (-0.35%)	⬇️
go/common/badger/migrate.go	0.00% <0.00%> (ø)
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-8.42%)	⬇️
go/storage/api/root_cache.go	80.00% <0.00%> (-4.45%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	66.19% <0.00%> (-3.34%)	⬇️
go/worker/storage/service_external.go	54.28% <0.00%> (-2.86%)	⬇️
go/runtime/tagindexer/tagindexer.go	66.66% <0.00%> (-2.30%)	⬇️
go/runtime/host/sgx/sgx.go	71.42% <0.00%> (-2.20%)	⬇️
go/storage/mkvs/cache.go	83.91% <0.00%> (-2.02%)	⬇️
go/storage/client/client.go	74.77% <0.00%> (-1.77%)	⬇️
... and 18 more

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