Refactor pgbench results output

Refactor pgbench results output

The idea is that one results document represents a test of one
particular commit.  Then it contains the results of all the tests run
against that commit.  I think it makes more sense to restructure the
pgbench results to be like:

{
    "pgbench": {
        "ro": {
            "results at a scale factor": {
                "number of clients": {
                    "std": 4389.727756305762,
                    "metric": 41390.590287,
                    "median": 43137.716637,
                    "results": [
                        {
                            ... pgbench results of a test ...
                        }
                        ...
                    ]
                },
                ....
        },
        "rw": {
            ... read write test results ...
    },
    ... other tests results
}