Return Path::Null() instead of dying from assertion failure.

Fixes busy loop when USB sound device gets unplugged (Mantis bug
#3824).

Use LockSynchronousCommand() instead of SynchronousCommandLocked().
Fixes regression from commit ef663810 (dead lock due to cond_wait with
unlocked mutex).

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.

The MPD core will add this as a prefeix.

Implement Mantis ticket 0003646.

Reduce the TagItem overhead.