Improve circuit breaking on aggregations

[jpountz]

Unfortunately circuit breaking on aggregations only catches a small subset of out-of-memory errors. I was talking with @dakrone recently who had the interesting idea to also count how many aggregators we create and use it for circuit-breaking. I think it would be a good heuristic to avoid out-of-memory errors in the deeply-nested aggregation case that we often see.

[dakrone]

When I discussed this with @jpountz we talked about potentially adding this to the constructor of AggregatorBase to increment when it is created (since we can get the breaker from the aggregation context) and decrement it in the close method, which I think is a neat idea to try.

Part of this would be determining the "weight" of a single AggregatorBase, then we could have a separate BucketBreaker where the overhead could be tuned for specific use cases (as a mulitplier of the class size).

Thoughts and heap dumps welcome :)

[dakrone]

If anyone is curious about this, I'm working on a branch with it here: master...dakrone:bucket-circuit-breaker