journald: don't employ inner loop for reading from incoming sockets

journald: don't employ inner loop for reading from incoming sockets

Otherwise, if the socket is constantly busy we will never return to the
event loop, but we really need to to dispatch other (possibly more
high-priority) events too. Hence, return after dispatching one message
to the event handler, and rely on the event loop calling us back
right-away.

Fixes #125