In our new `PerLevelDataLoaderDispatchStrategy`, we incorporate the
level of the current field (through the `ResultPath`) to detemrine when
we should dispatch `DataLoader`s. For:

 - a deep enough level
 - a large number of fields

this can become quite taxing.

To optimise this, we memoize the level by calculating it in the constructor,
which should have a negligible cost.