Re-introduce hybrid repo, add some tests for it

[NeilMountford]

This PR:

• Re-introduces the hybrid repo, fixing an error where requesting it from DI would always fail as it wasn't registered (but the type was public)
• Re-introduces the GetSnapshotWithCatchupExpensively method. I think this is useful for reading a single entity where the stream could end up having a lot of events in it
• Adds some tests to give the hybrid repo some basic coverage, as it currently had none
• Adds a summary section to the hybrid repo advising on when and where it is suitable