Memory usage and performance monitoring

We should monitor changes in memory usage and performance over time. Something along the lines of https://arewefastyet.com/.

This depends on the existence of a benchmark suite (see https://github.com/fullstaq-labs/fullstaq-ruby-umbrella/issues/9).

But a benchmark suite alone is not enough: we need to store historical data so that we can compare with the past. That's where monitoring comes in.