echo-cancel: Use volume sharing by default

echo-cancel: Use volume sharing by default

Uses the shared volume infrastructure by default with an option to
fallback on the old pretend-volume-sharing-that-kind-of-works if someone
wants it that way.

Users who keep left != right (or any sort of unbalanced channel volumes)
will likely want to disable shared volumes since it will cause their
master sink/source volume to be balanced.

This really isn't a very pleasant scenario since users would need to
manually set up echo cancellation in their config for this (until we
have a way to store module configuration). That said, the majority case
benefits from the volume sharing, so let's not wait for the
configuration infrastructure to be ready to use this.