IntoIterator did not work as expected anyway and would require usage of
AsRef, which then breaks type inference for empty slices for example.

stop should only really fail if everything is broken, but we can always
recover somehow here.

Only Speex, AAC and H264 are missing now, which require a little bit
more work.

This is not too nice now as we have None values to unwrap, but the code
is at least less convoluted.

It has the needed changes now

Don't lock the URI mutex all the time while {Sink,Source}::{start,stop} is called but only when needed