Move storage/mkvs/urkel to just storage/mkvs

[kostko]

Fixes #2657

The MKVS implementation has been changed from the initial "Urkel tree" structure
and it is no longer an actual "Urkel tree" so having "urkel" in its name is just
confusing.

[codecov]

Codecov Report

Merging #2792 into master will decrease coverage by 0.58%.
The diff coverage is 56.60%.

@@            Coverage Diff             @@
##           master    #2792      +/-   ##
==========================================
- Coverage   62.62%   62.04%   -0.59%     
==========================================
  Files         389      386       -3     
  Lines       37157    37095      -62     
==========================================
- Hits        23271    23016     -255     
- Misses      10924    11055     +131     
- Partials     2962     3024      +62     

Impacted Files	Coverage Δ
go/oasis-node/cmd/debug/byzantine/storage.go	64.51% <ø> (ø)
go/oasis-node/cmd/debug/storage/storage.go	45.08% <ø> (-3.47%)	⬇️
go/oasis-node/cmd/storage/benchmark/benchmark.go	3.92% <ø> (ø)
go/runtime/registry/storage_router.go	65.00% <ø> (ø)
go/storage/api/api.go	76.00% <ø> (ø)
go/storage/api/grpc.go	70.56% <ø> (+0.66%)	⬆️
go/storage/client/client.go	78.00% <ø> (ø)
go/storage/database/database.go	77.27% <ø> (-1.82%)	⬇️
go/storage/metrics.go	84.78% <ø> (-3.27%)	⬇️
go/storage/mkvs/cache.go	62.97% <0.00%> (ø)
... and 88 more

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[pro-wh]

alright, as long as we don't have other implementations in the future