Multiple nighthawk output aggregation support

If a consumer needs to saturate a very large backend, perhaps one running on multiple machines, that consumer may want to start multiple nighthawks in order to adequately saturate it. If they do so, combining the outputs together may prove to be difficult. Looking for a solution so that they can combine multiple results together into one useful output proto.

Possible Solution:
Allow nighthawk to be aware of multiple services running, with one service acting as the master and the rest acting as "remote workers." The workers could then stream their output directly to the master, which could aggregate the statistics together into one useful output proto for the consumer.