[refactor] split keeper funcs/interface into separate files

This does...

1. restructures the keeper into separate files with separate interfaces, which are combined to create the Keeper interface.
2. moves the dummy/mocks structs/interfaces into _test.go files. This is so they don't get compiled into our binaries, as well as ensures we cannot use dummy kvstore in non-test files.

There are no logic/code changes. Just restructuring.