Upgrades package improvements:

[ValarDragon]

We need to make the following improvements to our upgrades packages:

Define a type alias for Create Upgrade Handler

func CreateUpgradeHandler(
	mm *module.Manager,
	configurator module.Configurator,
	keepers *appkeepers.Keepers
) upgradetypes.UpgradeHandler {

Make an interface with four methods, describing an upgrade:

type Upgrade interface {
  UpgradeVersion() String // e.g. v7
  UpgradeHeight() int64
  CreateUpgradeHandler(mm, configurator, keepers) upgradetypes.UpgradeHandler
  StoreUpgrades() store.StoreUpgrades
  
  // Perhaps should be a separate interface? It gets called in begin block once, to do some quick migration in case of a hard fork not going through upgrades module.
  RunForkLogic(ctx) 
}

Then we just make a list of upgrader structs in the app.go, and have a standalone routine that registers the handlers, beginblock & store loader updates.