journal-gatewayd: timeout journal wait to allow thread cleanup

When a client connects with follow=1 and then disconnects we can get
stuck in sd_journal_wait indefinitely if no journal messages are logged.
Every time a client does this another thread is allocated and these
continue to stack until either a journal message is logged or we run out
of mapping to put a stack in.

By adding a timeout if we don't see any journal messages in that timeout
we will simply pop back out to microhttpd which will sanity check the
connection for us and if it is still connected pop us back into the wait
for more journal messages.