Use Clock#realTime for Open#startedAt

[peterneyens]

I was trying to debug an issue where a circuit breaker appeared to be stuck open and got confused by the started timestamp.

The documentation says:

startedAt is the timestamp in milliseconds since the epoch when the transition to Open happened

But with monotonic / System.nanoTime() we get an arbitrary duration.

The Timestamp comment also says:

Type-alias to document timestamps specified in milliseconds, as returned by Clock.realTime

[peterneyens]

In CircuitBreaker#of we could use cachedRealTime by passing the F[FiniteDuration] to SyncCircuitBreaker, but we can't do that in CircuitBreaker#in (we would need F[G[FiniteDuration]].