Cleaner sei/app

[Anmol1696]

Overview

Create app/upgrades package to have all the upgrades in one place with versions and abstract out all keepers to app/keepers to make the the app/app.go cleaner and more maintainable.

Problem Definition

app/app.go is too big file.

Proposal

split app/app.go into app.go, upgrade/, keepers/ , module.go

ref: osmosis/app provenance/app