convert DAGService to an interface

Here is DAGService  conversion, leaving the spiltter as-is.

Looking at BlockService, it would makes sense to interface that too, but for me at this point I'd rather have a RPC backed DAGService because its a much more naive way to add objects.

I would perhaps like to run multiple BlockExchange nodes on the same datastore, so that I might publish objects to different networks without exposing the relationship between my different addresses.
There is alot more that goes into that, but its something I've been thinking about.


been wanting to do this forever...