Make ConjunctionDISI package-private and add ConjunctionUtils factory class

[romseygeek]

ConjunctionDISI is really an internal implementation of DocIdSetIterator,
and would ideally be package-private. However, it is used in a few
other places:

• directly in ConjunctionSpans
• as a utility in the facet and join modules

This commit refactors Spans to use their own copy of ConjunctionDISI,
and adds a public helper class ConjunctionUtils that allows easy
intersection of iterators for use by other modules. This means that
ConjunctionDISI itself can become package-private. It also removes
a reference to Spans from core classes, which will make it easier to
migrate Spans to the queries module.

[jpountz]

LGTM. Let's add tests for the Spans fork of ConjunctionDISI?

[mikemccand]

+1 to this change -- looks great! But:

refactors Spans to use their own copy of ConjunctionDISI

it looks like you didn't have to do this (fork / make a full copy of ConjunctionDISI for spans)? It looks like spans just also use the new (public) ConjunctionUtils, which provide (safe) public access to the package-private ConjunctionDISI?

[romseygeek]

It looks like spans just also use the new (public) ConjunctionUtils, which provide (safe) public access to the package-private ConjunctionDISI?

Unfortunately no, if we do this then Spans don't use two-phase iteration any more. What might work instead though would be to make ConjunctionDISI.createConjunction(List<DocIdSetIterator>,List<TwoPhaseIterator>) public through ConjunctionUtils - I'll give that a try and report back!

[romseygeek]

We need to add three more methods to ConjunctionUtils but it does mean that we can remove the specialised Spans class, so I think it's a win. It should also be possible to re-use these for intervals as well, which have a similar specialised implementation.

[romseygeek]

I've pushed the intervals change as well, turns out it's very simple and allows us to remove more duplication.

[mikemccand]

Looks great, thanks @romseygeek!