We currently pass a SearchContext around to share configuration among
FetchSubPhases. With the introduction of runtime fields, it would be useful
to start storing some state on this context to be shared between different
subphases (for example, stored fields or search lookups can be loaded lazily
but referred to by many different subphases). However, SearchContext is a
very large and unwieldy class, and adding more methods or state here feels
like a bridge too far.
This commit introduces a new FetchContext class that exposes only those
methods on SearchContext that are required for fetch phases. This reduces
the API surface area for fetch phases considerably, and should give us some
leeway to add further state.
63afc61b08