runtime/storage/mkvs: Add method for checking local key existence

[jberci]

No description provided.

[codecov]

Codecov Report

Merging #2938 into master will decrease coverage by 0.04%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master    #2938      +/-   ##
==========================================
- Coverage   68.41%   68.36%   -0.05%     
==========================================
  Files         361      361              
  Lines       35220    35231      +11     
==========================================
- Hits        24095    24085      -10     
- Misses       8021     8035      +14     
- Partials     3104     3111       +7     

Impacted Files	Coverage Δ
go/oasis-node/cmd/genesis/genesis.go	52.40% <27.27%> (-0.65%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
go/common/cbor/codec.go	78.37% <0.00%> (-5.41%)	⬇️
go/worker/common/p2p/p2p.go	61.71% <0.00%> (-4.51%)	⬇️
go/consensus/tendermint/apps/registry/registry.go	62.16% <0.00%> (-4.51%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
go/runtime/transaction/transaction.go	74.54% <0.00%> (-3.64%)	⬇️
go/storage/client/client.go	75.33% <0.00%> (-2.65%)	⬇️
go/storage/mkvs/iterator.go	83.12% <0.00%> (-2.50%)	⬇️
go/consensus/tendermint/roothash/roothash.go	66.21% <0.00%> (-2.37%)	⬇️
... and 21 more

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