tools: perf: make the result files more compact

[janekmi]

Currently, when benchmarking is done a few JSON files come to existence e.g.:

.
├── apm_dram_vs_pmem.json
├── apm_pmem_vs_gpspm_pmem.json
├── ib_read_vs_rpma_read_dram.json
├── mix_rand.json
├── mix_seq.json
├── report.json
└── rpma_read_dram_vs_pmem.json

They are prepared for further processing by other scripts (report_figures.py, report_create.py). But as text files, it may be desirable for the end-user to read them directly. This may not be convenient because data points inside them are pretty spread out.