multi source support

This series adds multi-source support to the server. Source determination is provisional, and based just on channel prefix (e.g. everything before the first colon in the channel name), but it's a starting point to work off of.

The commits in the series are atomic, so the series shouldn't be squashed. The series includes various cleanup patches, but from a high level it flows as follows:

• rename the server class itself to be ArrakisFlightServer.
• modify the server class to take an initialized backend as argument. This intended to help make a cleaner interface between server and backend.
• make backends inherit from traits.ServerBackend, to allow specifying common code for every backend in a convenient way.
• have backends specify which sources they are providing.
• add multi-source support to the server.
• allow specifying source: location maps from the command line
• support information-only servers, which do not themselves have backends.