Consider output restructuring

Nighthawk currently defines its output as follows:

```proto
message Result {
string name = 1;
repeated Statistic statistics = 2;
repeated Counter counters = 3;
}

message Output {
google.protobuf.Timestamp timestamp = 1;
nighthawk.client.CommandLineOptions options = 2;
re
```

Statistics and counters have an id property. Consumers may become reliant on a specific id to be emitted.

A concrete example arose in the PR that adds an output transform for Fortio's format.
That relies on a) the aggregated global results to be around, and b) the a statistic with id `benchmark_http_client.request_to_response`.

Context: https://github.com/envoyproxy/nighthawk/pull/168/files/932814f6ee21ae4e2670fecfb42f80bbfd623afb#r341337505