Add KrimsonSubscriptionGapTracker

[gkinsman]

When creating projections in Krimson, its necessary to be able to ensure that a processor has processed all available events in an existing stream. This PR adds an interface IHasCaughtUp that can be used from anywhere DI is available to determine whether a processor with a given client id has caught up.

This solves an issue with existing usages of Krimson:

• Currently it's only possible to check whether a projection is up to date by checking KrimsonProcessorContext.HasCaughtUp which is only available when a message is received, whereas apps need to be able to perform this check on empty topics as well.

[RagingKore]

Added capability to check the gaps themselves and renamed the component.

[RagingKore]

Can merge as soon as you check the changes.

[gkinsman]

Looks great, nice one! 👍