Estimate and print overhead of clocks used by KV-Engine

As seen in a recent customer issue, the way the system clock is configured and the overhead in reading it can have a measurable impact on the performance of KV-Enigne.

It would be of use to print an estimate of the overhead of reading the two clock sources KV-Engine uses:

• Fine clock (std::chrono::steady_clock) which is typically capable of nanosecond prevision.
• Coarse clock (folly::chrono::coarse_steady_clock) - which is only capable of millisecond precision but can be much cheaper to read.