refactor: moving keepers to a new struct

[SebastianElvis]

Closes #699 , step 1 of #698

This PR refactors the BabylonApp to move all keepers into a new struct, including

• Creating a new struct AppKeepers that includes all keepers, under app/keepers. BabylonApp comprises AppKeepers such that no interface is changed.
• Moving keeper initialisation into AppKeepers.InitKeepers function. The function is parameterised such that all config entries remain in app/ folder.

This is in line with Osmosis' approach, for formalising software upgrading and for modularity. See #698 for more details.

[SebastianElvis]

I am not sure whether this refactoring is neccessary to achieve #698 goal, but is nice to see less code in app.go so LGTM!

Not strictly necessary, but

• It allows the Osmosis code to work out of the box in Babylon, and

• All CreateUpgradeHandler will have the same function signature

  https://github.com/babylonchain/babylon/pull/703 gives an intuition on how is the AppKeepers used