A little of coordination for Rosettacode

[Stanislav Blinov]

I forgot to note that both synchronized {} blocks should also be 
synchronizing on the same mutex. Right now it's two different 
critical sections, so a race is still possible, i.e. while one 
thread is printing the other may be removing the first printer. 
Run the code several times and you'll no doubt stumble upon it.

The mutex could be emulated with a shared bool and 
std.atomic.cas(). That would get rid of synchronized{} blocks and 
would potentionally be faster.