Supervised Buffer implementation

[cpjulia]

This PR introduces the velocypack part of the supervised buffer, which is a type of buffer used together with velocypack in query processing to account memory automatically instead of having to do it by hand in every function, so everything's centralized in one place. Yet, one must still avoid accounting for memory twice inside the code for now (to be changed)

[mchacki]

LGTM, no production change in this repository.

[mchacki]

Correction:

I think you need to make "clear" virtual as well, it can mess with the capacity in one branch.

We also need to make sure that move works as intended with respect to memory accounting