The bpd test bind a socket in order to test the protocol implementation. When
running concurrently this often resulted in an attempt to bind an already
occupied port.

By using the port number `0` we instead let the OS choose a free port. We then
have to extract it from the socket (which is handled by `bluelet`) via
`mock.patch`ing.

Under some circumstances (maybe under MS Windows?) local objects can't be
pickled. When `start_server` is a local this causes a crash:
https://ci.appveyor.com/project/beetbox/beets/builds/25996163/job/rbp3frnkwsvbuwx6#L541

Make `start_server` a freestanding function to mitigate this.

Close sockets in `finally`-clauses only after they have actually been created.

Use a `multiprocessing.Queue` instead of a `multiprocessing.Value` to avoid the
manual polling/timeout handling.

TODO: Strangely Listener seems to be constructed twice. Only the second one is
used. Fix that and then remove the code working around it.

When setting up bpd tests, two servers are startet: first a control server, then
bpd. Both send their assigned ports down a queue. The recipient only needs bpd's
port and thus skips the first queue entry.

…tbox#3330

Add a changelog entry asking plugin developers to report any further occurrences
of this failure.