supernova: plugin interface - guard access to rt-pool

plugins may access the rt memory poll from multiple threads, so we need to
protect the API.
guarding the complete rt memory pool would introduce some unnecessary
locking when dispatching osc commands.

Signed-off-by: Tim Blechmann <tim@klingt.org>
supernova: lazily initialize synths

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: In - cleanup

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: IOUgens - prevent buffer overflow

this patch introduces some safety guards to avoid writing data to
non-existing audio busses

Signed-off-by: Tim Blechmann <tim@klingt.org>