go/epochtime: add WatchLatestEpoch method

[ptrus]

Fixes: #2862

Pubsub framework is extended to support subscriptions based on bounded
ring channels. Bounded subscriptions are used in the new epochtime
WatchLatestEpoch method. The method is similar to the existing WatchEpochs
method, with the change that unread epochs get overridden with latest
epoch.

Registration worker is changed to use the WatchLatestEpoch method to
prevent trying to register for old epochs in case the worker falls
behind.

XXX: any other places where using a ring channel (e.g. skipping past/outdated channel items) would be preferable?

[codecov]

Codecov Report

Merging #2876 into master will increase coverage by 0.04%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master    #2876      +/-   ##
==========================================
+ Coverage   67.62%   67.67%   +0.04%     
==========================================
  Files         344      344              
  Lines       33623    33652      +29     
==========================================
+ Hits        22739    22773      +34     
+ Misses       7974     7946      -28     
- Partials     2910     2933      +23     

Impacted Files	Coverage Δ
go/epochtime/api/api.go	33.33% <ø> (ø)
go/epochtime/tests/mock_tester.go	82.97% <66.66%> (-5.60%)	⬇️
go/common/pubsub/pubsub.go	91.13% <100.00%> (+0.86%)	⬆️
go/consensus/tendermint/epochtime/epochtime.go	87.50% <100.00%> (+0.93%)	⬆️
...sensus/tendermint/epochtime_mock/epochtime_mock.go	73.61% <100.00%> (+0.94%)	⬆️
go/consensus/tendermint/keymanager/keymanager.go	86.74% <100.00%> (ø)
go/consensus/tendermint/registry/registry.go	66.92% <100.00%> (ø)
go/consensus/tendermint/roothash/roothash.go	67.90% <100.00%> (ø)
go/consensus/tendermint/scheduler/scheduler.go	78.30% <100.00%> (+1.88%)	⬆️
go/runtime/committee/nodes.go	80.39% <100.00%> (ø)
... and 32 more

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[kostko]

I think this looks good, just a minor cosmetic comment. I'll think about other use cases for ring channels, but that shouldn't block merging this PR.