rtpfunnel: srccaps thread-safety fixes for twcc case
commit 0bedd08a:
rtpfunnel: copy caps before sending them in a caps-event
Reason being we don't want downstream to own a ref to our
internal caps.
commit bc4031e8:
rtpfunnel: protect internal srccaps with lock
These are modified from sink pad event handlers, so
could be accessed from multiple threads at the same
time.