[prism] Using Real Time, when operating in "production" mode.

[lostluck]

Set processing Time timers and ProcessContinuations to be scheduled via a Real Time clock for a "quasi production" mode.

Requires per-pipeline configuration to be plumbed into the Element manager to toggle this line:

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go#L1640