go/upgrade: ensure upgrade handler exists

[ptrus]

Also changes:

• PushStage(api.UpgradeStageStartup) happens only after a successful stage completion
• removes RunningVersion
  • we already ensure that we are running upgrade-compatible version
  • and i think switching the version mid-upgrade (e.g. fixing a migration bug after a failure) should be fine, as long as the version stays compatible
• removes SubmittingVersion
  • this prevents changing binary while an upgrade is pending. This should be allowed - e.g. a compatible bugfix while an upgrade is pending.

[codecov]

Codecov Report

Merging #3768 (acae8a2) into master (c5aec61) will decrease coverage by 0.01%.
The diff coverage is 56.75%.

@@            Coverage Diff             @@
##           master    #3768      +/-   ##
==========================================
- Coverage   67.31%   67.29%   -0.02%     
==========================================
  Files         401      401              
  Lines       40000    40004       +4     
==========================================
- Hits        26925    26922       -3     
- Misses       9297     9301       +4     
- Partials     3778     3781       +3     

Impacted Files	Coverage Δ
go/upgrade/api/api.go	92.30% <ø> (ø)
go/upgrade/migrations/migrations.go	63.63% <42.85%> (-3.04%)	⬇️
go/upgrade/upgrade.go	58.82% <58.62%> (-2.66%)	⬇️
go/upgrade/migrations/dummy.go	78.57% <100.00%> (+0.79%)	⬆️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	69.23% <0.00%> (-7.70%)	⬇️
go/worker/common/committee/runtime_host.go	64.28% <0.00%> (-6.25%)	⬇️
go/worker/keymanager/handler.go	69.38% <0.00%> (-6.13%)	⬇️
go/common/cbor/codec.go	76.47% <0.00%> (-5.89%)	⬇️
go/consensus/api/submission.go	69.01% <0.00%> (-5.64%)	⬇️
... and 30 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5aec61...acae8a2. Read the comment docs.