Remove multi-activation grains from Catalog and ActivationDirectory

[ReubenBond]

This PR removes multiple activation concepts from the Catalog/'ActivationDirectory` as part of #6880.
To do that, I needed some way to deal with the last remaining "multi-activation" thing in our system: stateless worker grains.

To allow [StatelessWorker] grains to be represented by a single activation in the Catalog/ActivationDirectory, I added a new class, StatelessWorkerGrainContext, which implements IGrainContext for grains marked as [StatelessWorker].
Since a stateless worker can have multiple activations with the same grain id, StatelessWorkerGrainContext manages a collection of regular ActivationData instances and delegates message handling to them.

• ActivationCollector now relies on ICollectibleGrainContext, rather than referencing ActivationData directly.
• Properties on ActivationData which are common to all activations of a particular grain type were moved into GrainTypeSharedContext. This reduces the weight of each activation and makes it easier to implement StatelessWorkerGrainContext since it can delegate common functionality to that shared class.
• Orients ActivationDirectory around a GrainId to IGrainContext mapping, rather than ActivationId to ActivationData (which had additional collections to map GrainId). All notion of multiple activations per GrainId was removed from ActivationDirectory.
• Message.IsNewPlacement was removed, since it's no longer necessary.

The purpose is to:

• Generalize IGrainContext further in useful ways, so that other implementations can be added for different scenarios
• Further the work required to remove multiple-activation support so that we can remove the need for ActivationId (Remove ActivationId #6880)