Instead, abort and join previous thread (if any), just before creating
a new one.
Abort/join the last one when destroying the module.
Note: we may end up with a dangling (non-joined) thread for a
while (for example, when state goes from playing -> paused/stopped).
It will however be joined eventually; either when state goes to
'playing' again, or when the module is destroyed.
This allows us to update the elapsed field without having to talk to
MPD.
In particular, it fixes a bug where we reported the same elapsed value
when we we're updated due to *other* modules calling bar->refresh().
