BidirMMapPipe: silence message about mmapping strategy by default

Since this class has been used for quite some time now without major
trouble, it doesn't seem imprudent to hide the status messages about the
mmapping strategy by default. Since I have had requests to that end,
this is what I do with this commit. BidirMMapPipe gets two static
methods which allow toggling the message back on if the user so wishes.