[DON'T MERGE] feat(x/gov): add governors

[giunatale]

• You can only delegate to a single governor, and 100% of your governance VP or nothing, but you can always override the vote.
• Governors election is done on "estimated" VP exactly like validators, so only top X (configurable param) active governors by VP are counted for voting
• Governors can set their status between active and inactive but can change it only once every 4 weeks. Active governors can be elected (inactive governors are skipped) but the base account has automatically a self-delegation of all its VP to the governor and cannot redelegate. Inactive governors can re-delegate.
• Governors VP is "estimated" (I think it's an exact computation as I think I cover everything) using x/staking hooks so when slashing/bond/unbond/redel happen it triggers a VP update.
• It should be ok computationally and also ok in how data is stored for easy retrieval, however maybe not optimized for storage itself as there is the need for a couple of additional indexes. Need to also check how much the governor VP computation is accurate because it might allow to further simplify VP computation (skip part of it) in tally
• I also added an invariant but I am not sure how that impacts performance
• Currently missing all tests, code is also itself untested, it was a weekend project. No guarantees on quality or if it works at all (or as intended).

Based on discussions as recorded in https://gist.github.com/giunatale/95e9b43f6e265ba32b29e2769f7b8a37

EDIT:

• Governors need to have at least minSelfDelegation (param) tokens bonded in x/staking to be elegible for tally (active governors always require to "self-delegate" the governance VP from their base account)
• Added another invariant on governors delegations consistency with cumulative virtual validator shares stored

[giunatale]

An alternative system that does not make use of the aggregate VP is also available at:

• https://github.com/giunatale/atomone/tree/governors-no-aggregate-vp
• x/gov: remove aggregate VP for governors giunatale/atomone#1 (for a diff with this branch)

In brief:

• It removes the aggregate VP (and related indexing) for governors, so no need to loop over all involved governors in the slashing hook (which is removed here)
• Governor election is simplified to happen only among governors that voted, so if a governor had a lot of delegations but did not vote it basically gave up its spot. Actually since there is no need to limit the number of governors with this system the max governors param is removed. Only account for active governors that voted and meet the minimum self delegation requirement.
• Since there is no aggregate VP number, the optimization for quorum is not possible