Session management

[kayabaNerve]

We've now discussed session management as part of #145, #146, and #187. This issue is meant to document the strategy for it.

Also, to clarify session rotation will not be included in protonet 0.

The staking pallet allows allocating bond. As of now, the validator-sets pallet is the only planned allocator. Under our current planned designs, all validator sets will be selected to participate in Tendermint. That means validator-sets needs to tell the sessions pallet of all validators in sets.

This is increased in difficulty when we reach capacity, as we need to prioritize validators.

This is further increased in difficulty if we have multiple validator sets. We should be able to globally prioritize though, so long as each set still has one validator and is sufficiently collateralized (the latter property ensuring the former).

Tendermint does currently listen to the sessions pallet. It does need to be consolidated with validator-sets.

[kayabaNerve]

This issue is dated as hell. I believe it can be closed, wont-fix, under is current description. While it could be rewritten to still make sense, I'd rather make new issues as beneficial.