bpd: use plugin logger instead of global logger

[arcresu]

There was a code comment indicating that there was a plan to use the plugin logger instead of the global one, so this PR makes that change. It means we drop the --debug flag from the bpd command since the logger is now controlled by the normal beet -v verbosity flag. Also fixes up a debug message that was previously printed to stdout, and tweaks the format of the MPD protocol debug messages to make them a little easier to read/interpret. New look:

[...]
bpd: Building directory tree...
[...]
bpd: Finished building directory tree.
bpd: New client connected
bpd: server: OK MPD 0.13.0
bpd: client: command_list_ok_begin
bpd: client: status
bpd: client: currentsong
bpd: client: command_list_end
bpd: server: volume: 10
             repeat: 0
             random: 0
             consume: 0
             playlist: 0
             playlistlength: 0
             xfade: 0
             mixrampdb: 0
             mixrampdelay: nan
bpd: server: state: stop
bpd: server: list_OK
bpd: server: list_OK
bpd: server: OK

[sampsyo]

Awesome!! Yes, that has been a long-standing issue. 🚀 Thanks for cleaning this up!