Add support for JSON output format for perf_fast_forward results.
Add support for JSON output format for perf_fast_forward results.

The JSON output is arranged in a way that makes it easier to upload
results to ElasticSearch.
All the tests results are placed under the perf_forward_data_output/ directory
For test groups, we create separate subdirectories where we save results
from runs of tests in those groups.
For each test run, we store results in a separate file named:
    <dash-separated-param-list>.<run-number>.json
where
    <dash-separated-param-list> is a dash-separated list of parameters of the current
    test, e.g., 1-64 (for read-skip pattern).

    <run-number> is the number of run of this test with the specified
    parameters. This is needed as the same list of parameters can be
    used more than once (for instance, when cache is enabled).
    Those numbers start with 1, i.e., 1, 2, 3.

So, the path to a resulting JSON file may look like:
    perf_fast_forward_output/large-partition-skips/64-4096.1.json

Signed-off-by: Vladimir Krivopalov <dc9186a06078733915a6fcbab34e59120be2b484@scylladb.com>

scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla