minor cleanups.

signalfd failure check, possible leak on rate handling and some scheduling tunings for
the many sources cases, typically in relation to balancing across workers.
Get write lock on source earlier.

This may become a larger issue later with the stats link but acquire the lock
in the reserve so that the initialisation can complete before letting anyone
reference the source.
add a skip setting for mpeg processing, this should allow for non-parsing in odd setups

the case that has been raise involves audio/mpeg being specified for m4a files, while it should
be audio/mp4 in such cases, this patch allows for the content to pass through without doing
the constant rechecking and log reporting.