Add FetchAvailability result

[RichLogan]

This is 50% a proposal and 50% a PR.

FETCH_OK requires some attributes that an implementation needs to provide:

End Of Track: 1 if all objects have been published on this track, so the Largest Group ID and Object Id indicate the last Object in the track, 0 if not.
Largest Group ID: The largest Group ID available for this track.
Largest Object ID: The largest Object ID available within the largest Group ID for this track.

This allows the FetchReceived implementor to provide them, if the fetch can be resolved.

A similar API will be needed for JoiningFetch (for matching subscription lookup that needs to be implementor provided), so wanted to float this separately.

[RichLogan]

Given what was discussed today, will change this to be a more generic cache lookup callback.