Refactor MockQuerier

[chipshort]

The MockQuerier currently hides away its inner queriers. This forces us to forward all methods to make them available.
Exposing the underlying queriers directly would make it a little less boilerplatey.

#1593 (comment)

[chipshort]

@webmaster128 Is this something we want to do in 2.0?

That would allow us to remove all the forwarded function calls and just expose the inner queriers directly.
Otherwise, we'll have to keep the currently forwarded functions for backwards compatibility.

[webmaster128]

Yeah why not, let's give it a try and see how it would look like.