Bugfix: Block acceptance of new join jam requests until all local/remote music and art is loaded

[hwray]

We have a couple race conditions we can encounter in the process of joining a jam and exchanging music libraries:

• Both Master and Client phones need to load all their library metadata into the DB before accepting or sending join Jam requests, respectively.
• When the Master phone accepts a new Client into the Jam, the Master should receive, load, and re-broadcast the new Client's remote library and album art in its entirety before accepting another join jam request.

[JayThomason]

We are now blocking on exchange of music over the network (see #228), but we are not blocking on exchange of album art.