[wpilib] Timer: Add getLoopTimestamp()

[PeterJohnson]

This is updated at the start of each periodic function in TimedRobot, so is stable throughout the callback function execution.

TODO:

• Finalize naming (GetLoopTimestamp? GetPeriodicTimestamp? GetLoopStartTimestamp?)
• Provide access for testability in C++ (Java can use introspection)
• Update WPILib classes to use this where appropriate (should MathShared use it? should Timer use it?)

[oh-yes-0-fps]

You should mention that its loop start in the function ident I feel. When I first read getLoopTimestamp I was thinking time since the start of the loop. Also maybe an atomic bool that is flipped when a TimedRobot is initialized so you can more confidently fallback to getFPGATimestamp if the user isn't using a TimedRobot. This is especially important if its meant for libraries as the robot type the user uses is out of their control.

[thenetworkgrinch]

This would make sensor reading cache's so much easier

[PeterJohnson]

Closing in favor of #7378.