[WIP] Spanner benchmarking

[FirePing32]

fixes: #438

[c24t]

Looks like benchmarking.py is taken from @odeke-em's gist here: https://gist.github.com/odeke-em/929d308f1fb80a1661d2e3ccb6a7b58b.

As @odeke-em mentions in #438 (comment), what we're really interested in here is the extra latency that spanner-django adds on top of python-spanner. The python-spanner repo includes some YCSB benchmarks (added in googleapis/google-cloud-python#4539), but it's not clear that they're measuring the right thing, or that we can get a meaningful comparison in django.

Ideally we'd run these benchmarks as a periodic CI task and flag regressions automatically instead of checking the results into the repo.

[c24t]

I suspect we'll want the benchmarking script to run against the emulator since we're only interested in the performance of the ORM library here, and not the spanner service itself.

[c24t]

Closing as inactive, @FirePing32 feel free to reopen if you change this to address the comments above (run benchmarks in CI, against emulator, etc.).