Isolate handling of temporal context and metronome

[edevoogd]

Some of the more intricate pieces of ClockQuantizer were pulled out and wrapped in a TemporalContextDriver shim that abstracts the following items:

• Attach to an external ISystemClockTemporalContext, if one is provided
• Standardized exposure of ClockAdjusted and MetronomeTicked events
• Standing up an "internal" metronome if there is no external ISystemClockTemporalContext or if a provided ISystemClockTemporalContext does not provide a metronome.
• Quiescing support
  • Stopping any metronome events being fired from it. On top of this, if the TemporalContextDriver provides the "internal" metronome, timer resources are freed up while in quiescent state.
  • Retaining (buffering) only the last "clock adjusted" occurrence until the TemporalContextDriver is taken out of quiescing state
• Accounting for discrepancies in (absent) IAsyncDisposable support of System.Threading.Timer between TFMs

In order to clean things up a bit more, bool ISystemClockTemporalContext.ProvidesMetronome was changed to TimeSpan? ISystemClockTemporalContext.MetronomeIntervalTimeSpan. This allows implementers to not only specify the fact that an external metronome is being provided, but also its interval time-span.