Fix Scheduler implementation for ILP32 platforms such as watchOS

[christopherweems]

Fix Scheduler implementation for ILP32 platforms such as watchOS on arm64_32

Motivation:

Fix a bug in the Scheduler where converting the UInt64 nanoseconds value from a Duration value to the DispatchTimeInterval's Int associated value causes an uncaught overflow and crash on devices with 32-bit Int values.

Modifications:

Add DispatchTime.init(nowDelayedBy:), which computes the deadline and avoids casting to Int
Replace adhoc computations of Scheduler deadline with designated DispatchTime.init(nowDelayedBy:)

Result:

Prevents crash on ILP32 platforms when time delay exceeds a value that fits within a 32-bit Int

[ktoso]

@swift-server-bot test this please

[ktoso]

Unrelated test hang on 5.8 only: #1138

[ktoso]

Thanks, though I'm very surprised about using the cluster on a watch -- that's not a great ideas since it needs to perform heartbeats and active connections which would drain power quickly etc. It's not really a platform we support with the cluster.

Fix looks okey though, thank you!

[christopherweems]

Ahh my novice is showing -- thanks for taking a look. I'll look towards moving to another actor system, or rolling one, but it the mean time its really nice to be able to connect to the cluster system while I figure this stuff out. Thanks again!