Replace our own TimeAmount with Swift.Duration

[ktoso]

This is a must-fix; We no longer need to use our own time types.

The reason we had them to begin with was to not "leak" NIO's time types in our APIs. We no longer need to do so, or rather, should stop doing so as soon as possible.

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Use this everywhere instead: https://developer.apple.com/documentation/swift/duration

TimeAmount -> Swift.Duration

The purpose of our TimeAmount was to not "leak" NIO's TimeAmount, and we should keep true to that.

We should remove the time amount and use Duration everywhere.

Deadline -> ContinuousClock.Instant

I think for our purposes the https://developer.apple.com/documentation/swift/continuousclock will be good enough.

[ktoso]

I guess the conversions can be something like:

// I probably got the math wrong here ;-)

        let (s, `as`) = duration.components // seconds, attoseconds
        let sNanos = s * Int64(1_000_000_000)
        let asNanos = `as` / Int64(1_000_000_000)
        let (totalNanos, overflow) = sNanos.addingReportingOverflow(asNanos)
        
        let nanos: NIO.TimeAmount
        if overflow {
            nanos = .nanoseconds(.max)
        } else {
            nanos = .nanoseconds(totalNanos)
        }

we can absolutely just use .max when we get overflow; we never use such large values in practice.