Fall back musicbrainz server

I run a local musicbrainz server, but it is also on my main download/convert/trascode/media server and sometimes it bogs down.

When it does the local musicbrainz server is unreachable and beets can't find anything because it times out.

Would it be possible to add a backup server in the config for times like these? So when a local server times out it will reach out to the real musicbrainz server?

No file or test is named in the issue. Start by tracing how beets configures and contacts the local MusicBrainz server, including timeout handling; done means a configured backup server is tried when the local server times out and normal lookups still work when the local server responds.

Written by the indexing model from the issue text.