go/node: add consensus.backend flag

[ptrus]

Fixes: #1870

Done:

• removes trivial scheduler
• introduces consensus.backend flag

epochtime backend will be taken care of in #1793

TODO (after merging/before promoting):

• fix flags in chart
• fix flags in runtime-ethereum

[kostko]

Should we move all the tendermint backends somewhere under go/tendermint now that this is per-consensus-backend?

[Yawning]

Oh god please don't move the backends around till after I merge #1858.

[codecov]

Codecov Report

Merging #1878 into master will increase coverage by 1.84%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1878      +/-   ##
==========================================
+ Coverage   53.47%   55.31%   +1.84%     
==========================================
  Files         223      221       -2     
  Lines       21079    20333     -746     
==========================================
- Hits        11271    11247      -24     
+ Misses       8603     7893     -710     
+ Partials     1205     1193      -12

Impacted Files	Coverage Δ
go/registry/tendermint/tendermint.go	69.27% <ø> (ø)	⬆️
go/beacon/tendermint/tendermint.go	66.25% <ø> (ø)	⬆️
go/staking/tendermint/tendermint.go	60.11% <ø> (+2.89%)	⬆️
go/epochtime/tendermint/tendermint.go	0% <ø> (ø)	⬆️
go/tendermint/api/api.go	37.03% <ø> (ø)	⬆️
go/keymanager/tendermint/tendermint.go	38.27% <ø> (ø)	⬆️
go/roothash/tendermint/tendermint.go	64.12% <ø> (ø)	⬆️
go/scheduler/tendermint/tendermint.go	61.22% <ø> (+4.08%)	⬆️
go/ekiden/cmd/node/node.go	52.56% <100%> (ø)	⬆️
go/ekiden/cmd/genesis/genesis.go	12.77% <100%> (+1.82%)	⬆️
... and 22 more

---

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[codecov]

Codecov Report

Merging #1878 into master will increase coverage by 0.97%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1878      +/-   ##
==========================================
+ Coverage   53.47%   54.44%   +0.97%     
==========================================
  Files         223      222       -1     
  Lines       21079    20685     -394     
==========================================
- Hits        11271    11261      -10     
+ Misses       8603     8219     -384     
  Partials     1205     1205

Impacted Files	Coverage Δ
go/registry/tendermint/tendermint.go	69.27% <ø> (ø)	⬆️
go/beacon/tendermint/tendermint.go	66.25% <ø> (ø)	⬆️
go/staking/tendermint/tendermint.go	57.21% <ø> (ø)	⬆️
go/epochtime/tendermint/tendermint.go	0% <ø> (ø)	⬆️
go/tendermint/api/api.go	37.03% <ø> (ø)	⬆️
go/keymanager/tendermint/tendermint.go	38.27% <ø> (ø)	⬆️
go/roothash/tendermint/tendermint.go	64.12% <ø> (ø)	⬆️
go/scheduler/tendermint/tendermint.go	57.14% <ø> (ø)	⬆️
go/ekiden/cmd/node/node.go	52.71% <100%> (+0.15%)	⬆️
go/ekiden/cmd/genesis/genesis.go	11.27% <100%> (+0.32%)	⬆️
... and 10 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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