Optional monitors suggestion

[Martin Nowak via Digitalmars-d]

> If we didn't have to worry about being backwards compatible, 
> I'd definitely argue for the second solution. Java 
> compatibility is not a very strong argument in my opinion. 
> First, porting a Java application 1:1 is asking for  
> performance hazards (w.r.t. GC, ...) anyway. Second, the 
> no-synchronized-by-default design allows for clear error 
> messages that immediately suggest the correct fix (add an 
> attribute to the class declaration), and for mechanical 
> porting, Java classes could just be translated to 
> "@synchronizable class" or whatever.

I think deprecating the old behavior is the right choice here.