Previously we simply logged errors but never reported them to elements
or even to the user. Fatal errors are now properly reported.
Additionally proper connection closing is implemented based on EOS:
- dtlsenc: EOS will cause close_notify to be sent to the peer and only
if the peer also sent back close_notify we will forward the
EOS event.
- dtlsdec: EOS will be forwarded normally, this only means that the
unterlying transport was closed. On receiving a DTLS packet
containing close_notify, return EOS and send EOS downstream.
This way we will share threads with other DTLS connections if possible, and
don't have to start/stop threads for timeouts if there are many to be handled
in a short period of time.
Also use the system clock and async waiting on it for scheduling the timeouts.
