MEDIUM: stream: Hold a reference on the frontend during stream_free()

MEDIUM: stream: Hold a reference on the frontend during stream_free()

The frontend of a stream is dereferenced until the end of stream_free(), in
particular to flush the maximum amount of pools when the proxy is stopping.
But when the stream connectors are destroyed, a client applet attached to
the stream is released there, and it may free its internal frontend (created
owned by the applet), leaving the end of the function with a dangling
frontend.

Take a reference on the frontend before releasing the stream connector and
drop it at the very end of stream_free(), after the last dereference of the
frontend. For the frontends that are never released during the lifetime of
the process, this is a balanced pair of reference counter updates.

This will be needed by the next commits, adding a destroy callback to the
applet ops, called when an applet context is destroyed, which may release
the frontend of its applet stream.