Rewrite distance operator <-> into the specific distance function if existing index won't trigger

[Ngalstyan4]

Currently we properly detect and fail the query in cases when the distance operator <-> is used outside of index-search context

We only check for existence of the index, however, and we do not check whether the index will actually be triggered.
This means that on small tables search queries fill fail unless we set enable_seqscan=off. This is bad UX.

We can instead detect that index is not used and rewrite the query to trigger the right distance operator.

[gnehzza]

I'm currently working on this issue, to avoid someone else overlapping with me.