bpd: support MPD partitions

MPD has a partitions feature that allows different clients to interact with different "views" in MPD. Each partition has its own player queue and such. If I properly understand the feature this is intended to be used with a different output device for each partition so that the same server and music library can back multiple different outputs.

BPD currently only supports a single output, so for now this feature would be rather useless. If we add support for e.g. a streaming output like what MPD itself does, then this could become useful. It fixes #785

In terms of the implementation, BPD currently only supports a single `Server` instance with one set of state. We would need to be able to startup multiple `Server`s and understand what they need to share (e.g. `update` and persistent playlist commands should affect all instanced) and what is separate (player queue, output configuration).