go/consensus/tendermint/roothash: Only index the correct runtime

[kostko]

In case multiple runtimes were being tracked, the indexing process could
incorrectly set the last indexed round which could make it skip to index a
round.

[codecov]

Codecov Report

Merging #4351 (04db11b) into master (3f28a5c) will decrease coverage by 0.78%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #4351      +/-   ##
==========================================
- Coverage   69.38%   68.59%   -0.79%     
==========================================
  Files         411      411              
  Lines       47417    47418       +1     
==========================================
- Hits        32898    32527     -371     
- Misses      10516    10851     +335     
- Partials     4003     4040      +37     

Impacted Files	Coverage Δ
go/consensus/tendermint/roothash/roothash.go	69.64% <66.66%> (-3.29%)	⬇️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/storage/api/context.go	87.87% <0.00%> (-12.13%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.03% <0.00%> (-12.09%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
go/consensus/tendermint/apps/staking/auth.go	70.37% <0.00%> (-7.41%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-6.37%)	⬇️
go/storage/api/mux.go	56.34% <0.00%> (-6.35%)	⬇️
... and 47 more

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