Fix Clock for performance counter: Add currentTimeNanos() since module startup, retrievable via getMonotonicStartupTime(). (performance)

Settings two long fields in getMonotonicTime() and creating Instant and using Duration
for high-frequency counter is too expensive.

currentTimeNanos() subtracts the startup time from the current monotonic time and returns the
resulting duration in nanoseconds, which lasts for 292 years since module startup.
This satisfies performance counter requirements.

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