Aggregation Spouts to use shard request cache

[jnioche]

See https://www.elastic.co/guide/en/elasticsearch/reference/2.4/shard-request-cache.html

This would make the aggregations faster but depends on the value of the refresh interval. We'd need to change the logic around the nextFetchDate query so that we reuse an existing value unless it does not return any results. This alone would also make the search faster as we'd query a diminishing set of documents.

[jnioche]

Note: cache is disabled by default in ES 2.x but enabled in 5.x

[jnioche]

The logic is now to reset the nextFetchDate only when the number of buckets is 0. We'll refine the mechanism in #452