Add benchmarking framework

[Bronek]

Summary

Add support for (micro)benchmarking of rippled code segments. Ideally outside of unit tests, since running benchmarks in poorly, if at all, controlled conditions serves little purpose. As a minimum should report mean time of operation in ns, number of repetitions and/or standard deviation of the measurement.

Motivation

More systematic approach to performance optimisation of rippled

Solution

Add support for some microbenchmarking framework - either in house or something like https://github.com/google/benchmark

Paths Not Taken

Ad-hoc microbenchmarks scattered through the code