[apmbench/apmsoak] Allow scaling of load generation with meaningful benchmark results

[lahsivjar]

Using apm{bench|soak} in a distributed way to scale load generation would cause the benchmark results to be broken. We should handle this by introducing a mechanism to select a leader/coordinator. The leader will be in charge of collecting and reporting benchmark metrics while the non-leader nodes will generate load and communicate their state to the leader. The communication can be explicit (using a gossip protocol) or implicit (by monitoring benchmark metrics).