This commit allows coordinating node to account the memory used to perform partial and final reduce of
aggregations in the request circuit breaker. The search coordinator adds the memory that it used to save
and reduce the results of shard aggregations in the request circuit breaker. Before any partial or final
reduce, the memory needed to reduce the aggregations is estimated and a CircuitBreakingException} is thrown
if exceeds the maximum memory allowed in this breaker.
This size is estimated as roughly 1.5 times the size of the serialized aggregations that need to be reduced.
This estimation can be completely off for some aggregations but it is corrected with the real size after
the reduce completes.
If the reduce is successful, we update the circuit breaker to remove the size of the source aggregations
and replace the estimation with the serialized size of the newly reduced result.

As a follow up we could trigger partial reduces based on the memory accounted in the circuit breaker instead
of relying on a static number of shard responses. A simpler follow up that could be done in the mean time is
to [reduce the default batch reduce size](elastic#51857) of blocking
search request to a more sane number.

Closes elastic#37182

This commit removes the serialization of partial reduce in order to speed up the merges when
the batch reduce size is smaller than the number of shards in the request.
The estimation of the size of partial reduce is still based on the binary size (serialized form) but we
keep the full java object and estimate the size with a counting stream output.
Finally this change adds a benchmark for the reduce of nested terms aggs. This benchmark was used to
optimize the code in this PR.

… master.