Improve immediate measurements

[ritchiecarroll]

Adds HasMeasurementID function to see if immediate measurement cache has desired MeasurementKey

[StephenCWills]

FYI, I convinced Ritchie to close the PR, thinking that immediateMeasurements.MeasurementIDs.Contains() would be equivalent to HasMeasurementIDs(), but it turns out that's only true for Dictionary<TKey, TValue>. The ConcurrentDictionary<TKey, TValue> implementation actually creates a copy of the keys as an array and wraps it in a ReadOnlyCollection<TKey>. So in this case, it's much more efficient to use ContainsKey() directly.