MappedFieldType.rangeQuery should rewrite to an `exists` query when both bounds are open

[jpountz]

Queries that have both bounds open can be generated even if the user passed explicit bounds due to query rewriting. Unfortunately, queries that have both bounds open are still costly to execute, so we should look into using exists queries instead (or even match_all queries if all docs have a value for the field).

[AnnaGMiguel]

Hello @jpountz, I'm looking for my first contribution to elasticsearch.
I'm familiarizing myself with the code base.

[jpountz]

@AnnaGMiguel Sorry for the late reply, I just saw your message. If you are looking for help to get started, you can look at the RangeQueryBuilder class.