Add storage export debug subcommand

[kostko]

Fixes #1845.

Blocked on #1835.

TODO

• Merge with genesis dump tool to export all roots for the dumped roothash block(s).

[codecov]

Codecov Report

Merging #1847 into master will decrease coverage by 0.08%.
The diff coverage is 14.75%.

@@            Coverage Diff             @@
##           master    #1847      +/-   ##
==========================================
- Coverage   52.62%   52.54%   -0.09%     
==========================================
  Files         263      264       +1     
  Lines       26489    26550      +61     
==========================================
+ Hits        13941    13951      +10     
- Misses      11090    11141      +51     
  Partials     1458     1458

Impacted Files	Coverage Δ
go/common/namespace.go	59.09% <0%> (-17.38%)	⬇️
go/storage/mkvs/urkel/writelog/writelog.go	28.57% <0%> (-7.8%)	⬇️
go/ekiden/cmd/debug/storage/storage.go	6.07% <100%> (+1.04%)	⬆️
go/ekiden/cmd/debug/storage/export.go	13.72% <13.72%> (ø)
go/tendermint/apps/staking/staking.go	50.45% <0%> (+0.22%)	⬆️

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[Yawning]

This can be closed since #2495 has a newer stab at implementing this.