support filtered query

[newgene]

When the order of the hits and _score values do not matter, the filtered query can be preferred. There are several ways to make such queries:

https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-bool-query.html#score-bool-filter

We can potentially support this by introducing a filter parameter, which takes the same parameter as q, but pass as the filtered query. It can be combined with q as the query part as well.

[newgene]

alternatively, we can reuse q parameter, but use an "as_filter" flag to indicate if the query should be queried as a filtered query or not. In this case, no combination of q and filter, can only do either one.

We can discuss to see which is preferred, or even support both.

[zcqian]

Using the filter context may improve performance if it gets cached by Elasticsearch (which is, in the end, a maybe). Other than that, is there a specific reason that we would want to use the filter context instead of the query context?

In some cases we are already using both: for example in interval queries in MyVariant

[zcqian]

Another thing is we can't make this opt-out, but it brings little benefit if it's opt-in.

The user making the query probably doesn't care about the performance impact of the query.