LUCENE-8477: Automatically rewrite disjunctions when internal gaps matter

[romseygeek]

We have a number of IntervalsSource implementations where automatic minimization of
disjunctions can lead to surprising results:

• PHRASE queries can miss matches because a longer matching sub-source is minimized
  away, leaving a gap
• MAXGAPS queries can miss matches for the same reason
• CONTAINING, NOT_CONTAINING, CONTAINED_BY and NOT_CONTAINED_BY queries
  can miss matches if the 'big' interval gets minimized

The proper way to deal with this is to rewrite the queries by pulling disjunctions to the top
of the query tree, so that PHRASE("a", OR(PHRASE("b", "c"), "c")) is rewritten to
OR(PHRASE("a", "b", "c"), PHRASE("a", "c")). To be able to do this generally, we need to
add a new pullUpDisjunctions() method to IntervalsSource that performs this rewriting
for each source that it would apply to.

Because these rewritten queries will in general be less efficient due to the duplication of
effort (eg the rewritten PHRASE query above pulls 5 term iterators rather than 4 in the
original), we also add an option to Intervals.or() that will prevent this happening, so that
consumers can choose speed over accuracy if it suits their usecase.

[jimczi]

The change looks good to me. I am not sure if the rewriting should be the default option since minimum interval semantic is the basis of the paper and I have the feeling that we try to hide this from users. However I agree that missing matches can be problematic since users may never notice it so I am fine either way ;).

[jimczi]

can you add a comment explaining why we separate singleton source ?