Use clock_gettime for all performance timestamps

What does this MR do?

This MR adjusts the performance monitoring code to use Process.clock_gettime (thus clock_gettime(3)) instead of Time.now.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Using Time.now / Time.new adds more overhead than Process.clock_gettime, it also doesn't provide a way of getting timestamps in nanoseconds (which Process.clock_gettime does allow).

What are the relevant issue numbers?

None.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together