Add a way to remove specific feeds with all associated data

[boreq]

We need a mechanism for removing a specific feed with all associated data. This may for example include:

• blobs (if no other feeds reference them/the temporary on-demand want list doesn't reference them)
• contacts
• pubs
• etc

This is needed for #16 and more importantly #20. I am creating this issue to distinguish between adding a way to drop a feed and implementing the slightly more complicated case of cleaning up feeds described in #16.

[boreq]

This may be problematic after #33 is added as other clients may not understand that the local sequence goes back to zero - I think the JS code specifically doesn't accept that number going down for some reason.