StreamRestore: Cache values in writeChanges until receiving change from pa

pa_ext_stream_restore have only one function to change all its
properties, so we need to cache values between clients changing
StreamRestore properties and pulseaudio actually signaling the property
was changed.

Fixes correctly muting Notification Sounds stream in KCM when moving
slider to 0 value, because in that moment the KCM first sets volume to 0
and then immediately mute to true.

Differential Revision: https://phabricator.kde.org/D4674