Make UpdateConsensus to choose the proposer

Foundry host eventually has to use the 0th validator given by `UpdateConsensus`, as the proposer.
This will remove old CodeChain proposer selection logic https://github.com/CodeChain-io/foundry/pull/602#pullrequestreview-556853470.
And also, to express complicated staking rule considering tendermint process, we have to pass the current view (round) to the module, via `block_opened()`.

Locate the UpdateConsensus and block_opened entry points, then read the linked pull request review for the existing proposer-selection context. Trace how the current view is passed into the module and how validators are returned. Done means UpdateConsensus exposes the proposer through its validator result and the old host-side selection logic is removed.

Vom Indexierungsmodell aus dem Issue-Text verfasst.