cache volume inside PAStreams when trying to set it on an invalid one

non-PA phonon caches all and everything and also the volume. in particular
you can set the volume before reaching playing state (which in fact is
not supported by VLC nor PulseSupport). Yet from a convenience POV this
has actual use so this change introduces caching inside PAStream.

When PASupport gets a cb about sink changes it tries to update the volume
in our PAStream accordingly. Before doing so we now try to apply a
possibly cached volume using queued invoke method as right now it's not
clear whether it's safe to change the volume from inside the callback
thread.

I'd like to get this fix verified by at least some people using knotify4
before considering the issue actually resolved. WRT knotify simply trying
to change the volume setting inside the KCM will do (upon sound output
the notification volume in pavucontrol/kmix should update accordingly)

CCBUG: 321172