go/consensus/tendermint: Only reset executor pool after emitting block

[kostko]

Make sure to only reset the executor pool after any timeouts have been cleared
(e.g. when an empty block is emitted) as otherwise there could be a stale
timeout.

[codecov]

Codecov Report

Merging #4323 (a3f889a) into master (68fbd04) will decrease coverage by 0.07%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #4323      +/-   ##
==========================================
- Coverage   68.63%   68.56%   -0.08%     
==========================================
  Files         411      411              
  Lines       47370    47375       +5     
==========================================
- Hits        32511    32481      -30     
- Misses      10825    10853      +28     
- Partials     4034     4041       +7     

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/roothash/roothash.go	74.48% <28.57%> (-0.87%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/storage/api/context.go	87.87% <0.00%> (-12.13%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/runtime/host/protocol/connection.go	62.16% <0.00%> (-6.18%)	⬇️
go/consensus/api/submission.go	66.66% <0.00%> (-4.77%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	84.21% <0.00%> (-3.51%)	⬇️
go/sentry/api/grpc.go	56.25% <0.00%> (-3.13%)	⬇️
go/keymanager/client/client.go	76.03% <0.00%> (-2.48%)	⬇️
go/worker/storage/committee/node.go	75.99% <0.00%> (-1.80%)	⬇️
... and 19 more

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