go/consensus: Store runtime state roots in a single key

[kostko]

That makes it easier to construct proofs starting at the consensus state root
that prove what the state root of a specific runtime is. You can then chain
these proofs to prove something about runtime state.

[codecov]

Codecov Report

Merging #3782 (28586d0) into master (7d0d54b) will decrease coverage by 0.07%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #3782      +/-   ##
==========================================
- Coverage   67.32%   67.24%   -0.08%     
==========================================
  Files         402      402              
  Lines       40560    40569       +9     
==========================================
- Hits        27305    27281      -24     
- Misses       9410     9427      +17     
- Partials     3845     3861      +16     

Impacted Files	Coverage Δ
.../consensus/tendermint/apps/roothash/state/state.go	68.64% <40.00%> (-6.36%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
go/storage/api/root_cache.go	71.11% <0.00%> (-4.45%)	⬇️
go/storage/mkvs/tree.go	86.95% <0.00%> (-4.35%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	66.19% <0.00%> (-4.29%)	⬇️
go/consensus/tendermint/full/services.go	80.00% <0.00%> (-4.17%)	⬇️
go/consensus/tendermint/roothash/roothash.go	69.03% <0.00%> (-3.81%)	⬇️
go/runtime/host/sgx/sgx.go	71.42% <0.00%> (-3.30%)	⬇️
go/storage/mkvs/insert.go	87.67% <0.00%> (-2.74%)	⬇️
...consensus/tendermint/apps/scheduler/state/state.go	62.22% <0.00%> (-2.23%)	⬇️
... and 17 more

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