release v0.20.4

Fixes race condition when epoll_ctl() gets called after the socket has
been closed, which may affect a different socket created by another
thread meanwhile.

When rpc_reconnect_requeue() gets called from inside nfs_service(),
the NfsInputStream can stall completely because the old socket has
been unregistered from epoll automatically, but the new one has never
been registered.  Therefore, nfs_service() will never be called again.

This kludge attempts to detect this condition by checking
nfs_which_events()==POLLOUT.

https://bugs.musicpd.org/view.php?id=4081

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

GCC 4.9 has incomplete C++14 support.  Specifically, it doesn't allow
switch/case in "constexpr" functions.

Eliminate dependency on io_thread_get().

Make sure we're using the same EventLoop as the NfsManager.

Eliminate dependency on io_thread_get().

Eliminate dependency on io_thread_get().