Support epoch module and epoch staking

[edisonz0718]

Description and context

Currently, the validator set changes every block depending on the voting power. We want to introduce the concept of the epoch, which can be configured as a time window or a number of blocks. This gives us more flexibility to schedule periodical tasks that cannot be or don't need to be fit into the per-block schedule.

With the epoch, we will move our validator set change to each epoch instead of each block, which also give more stability to the system.

Suggested solution

cosmos-sdk v0.52 or osmosis's epoch module can be referenced. Another reference is Babylon's epoch staking design

Definition of done

Design and finalize the best approach in our case
Fully implement and test the solution