Now that the channel.DB no longer uses the ChannelGraph pointer, we can completely remove it as a member of channeldb.DB.
We also now use the graph DB's own optional functions. An instance of the graph is currently still passed to the channeldb's `CreateWithBackend` function. This will be removed in a later commit once the two have been completely disjoint.
This is a pure refactor commit. It moves over all the graph related CRUD code from `channeldb` to `graph/db`.
