Use disorder-choose to pick random tracks.

State and periodic activity handling is reorganized.  Periodic
activities started from main() are now handled in much more a
consistent manner.

The server will now check for new track to play once a second, and try
to add a new random track once every ten seconds.  In addition various
bits of play.c attempt these operations when immmediate response is
more appropriate, but are now freed from the responsibility of
ensuring that they occur at a reasonable frequency.

A couple of features have disappeared for now:
- disorder_track_random() is gone form the server API.  This has been
  useless for a while now.
- The 'gap' configuration item no longer works.  I'm not convinced
  it'll be missed; 0 is by far the most useful setting since otherwise
  album play is disrupted.