Replace epochtime with ticker

[ptrus]

Replace epochtime with a ticker-time. The general idea is that ticker will serve as a (configurable) base frequency. Everything currently relying on epochtime.WatchEpochs will be able to be configured as a multiplayer of the base frequency.

This will also enable: #1654
And deprecate: #1762

Details

• rename epochtime to ticker
• Extend api to include: WatchTicks(multiplyer uint64) (<-chan api.Ticker, *pubsub.Subscription)
• everything currently using WatchEpochs should be updated to use the configurable WatchTciks() (with configurable multiplyer)

[ptrus]

Additional thoughts/plan:

• remove the notion of epochs completely, instead there'll be compute/txnscheduler/compute/registry/...-intervals
• since there's no more notion of "global" epoch, we cannot rely on per "epochs" lists, instead the API's between services should be per "block heights" -> this requires Remove non-Tendermint backends #1799 as currently non-tendermint backends don't have notion of heights.

[ptrus]

Also: fold epochtime under the consensus.backend banner, and give epochtime/tendermint_mock the #1872 treatment

[kostko]

👍