Simplify BucketedSort/Teach BitArray a useful trick (backport #53199)

[nik9000]

Our lovely BitArray compactly stores "flags", lazilly growing its
underlying storage. It is super useful when you need to store one bit of
data for a zillion buckets or a documents or something. Usefully, it
defaults to false. But there is a wrinkle! If you ask it whether or
not a bit is set but it hasn't grown its underlying storage array
"around" that index then it'll throw an ArrayIndexOutOfBoundsException.
The per-document use cases tend to show up in order and don't tend to
mind this too much. But the use case in aggregations, the per-bucket use
case, does. Because buckets are collected out of order all the time.

This changes BitArray so it'll return false if the index is too big
for the underlying storage. After all, that index can't have been set
or else we would have grown the underlying array. Logically, I believe
this makes sense. And it makes my life easy. At the cost of three lines.

but this adds an extra test to every call to get. I think this is
likely ok because it is "very close" to an array index lookup that
already runs the same test. So I think it'll end up merged with the
array bounds check.