Keep track of installed number of probes to shortcut emission.
Allow NULL callbacks, this is useful for blocking probes.
Improve probe selection based on the mask, an empty mask for the data or the
scheduling flags equals that all probes match.
Add some more debug info.
Don't check the flushing flag in the probe callback handler, this needs to be
done before calling the handler.
Fix blocking probes.
Fix unit tests
Make a downstream element return an error after upstream has already
put all data into queue (including EOS). As such, upstream
will not be around to pick up the error, so it is up to queue to
act appropriately. See #589991.
Note there may be downstream fatal errors (e.g. negotiation) that do
not warrant an error message already having been posted.
