proxy: Allow to reuse proxysrc with a different proxysink

Problem: proxysrc could not be reused after disconnected, basically proxysrc-proxysink pair could only be used once, because a) after being disconnected and reconnected, elements remained in flushing state b) after reconnected proxy did not know it had to re-send caps

We've used the code with these changes for a long time, so it WAS tested and worked, although lately we've mainly phased proxysrc/sink out. Still, I think it's useful.