Add support for multi search

[jsvd]

Elasticsearch provides an _msearch endpoint to perform multiple searches in a single round trip.

Since the filters have both a filter(event) and a multi_filter(events) (link) api, we could ensure that each batch of events is searched in 1 operation.

This doesn't replace caching but will likely increase this plugin's performance by close to an order of magnitude, like we say with the http output's json_batch