alsa: rework batch handling a bit

By default, use a 512 period for batch and use 512 headroom
Add a property to disable special batch handling.

src/daemon/media-session.d/alsa-monitor.conf

@@ -58,6 +58,7 @@ rules = [
                 #api.alsa.period-size =         1024
                 #api.alsa.headroom =            0
                 #api.alsa.disable-mmap =        false
+                #api.alsa.disable-batch =       false
             }
         }
     }