app_mixmonitor: Turn on synchronization by default

The optional synchronization behavior created in
64906c4c is now the default for
MixMonitor.

* Add a new flag 'n' that allows for this behavior to be turned off

* Add a notice when the 'S' option is used indicating that it is no
  longer necessary

Change-Id: I158987c475cda4e1ff1256dd0daccdd99df568b4