Add TimePoint and use it in Timeout

[stIncMale]

This PR introduces internal API needed for JAVA-5076 (see #1166). I separated the change in this PR to make #1166 more focused on business logic.

The internal API introduced in the PR is intended to solve the following problems:

1. I need to measure elapsed time. System.nanoTime is such a tool, but it would have been better to have a tool that exposes types less generic than long, and values that are meaningful on their own (a value returned by System.nanoTime is meaningless on its own).
2. I need to measure elapsed time from the same instant that a Timeout was started at.

[stIncMale]

This method went to Timer.

[katcharov]

I am unsure about the approach - I did not review everything in detail, pending the main comment above.

[katcharov]

Just a couple of optional comments.

[vbabanin]

[Optional] According to the javadoc in TimePoint, we can replace TimePoint.now().durationSince(this) with the method elapsed() to avoid passing a parameter down function.

    @VisibleForTesting(otherwise = PRIVATE)
    long remainingNanos() {
        return Math.max(0, durationNanos - assertNotNull(start).elapsed().toNanos());
    }

[stIncMale]

The whole point of exposing remainingNanos for tests is so that tests could pass an arbitrary "now". If this were not needed, remainingNanos would not have been extracted from remaining. With elapsed, using an arbitrary "now" is impossible.

[vbabanin]

LGTM, just one optional comment