stm: feedback forwarding has to be atomic
stm: feedback forwarding has to be atomic

I think that a better place for the mutex would be the printing routine,
but I still hope we will get rid of threads in favor of coroutines.
So I keep all mutexes in Stm.

Ptival/coq,Ptival/coq,silene/coq,william-lawvere/coq,Matafou/coq,coq/coq,Ptival/coq,letouzey/coq-wip,ejgallego/coq,maximedenes/coq,ejgallego/coq,gares/coq,maximedenes/coq,ejgallego/coq,gares/coq,amintimany/coq,gares/coq,SkySkimmer/coq,JasonGross/coq,coq/coq,letouzey/coq-wip,letouzey/coq-wip,Matafou/coq,Lionel-Rieg/coq,ppedrot/coq,Matafou/coq,william-lawvere/coq,ppedrot/coq,Zimmi48/coq,ppedrot/coq,ppedrot/coq,Zimmi48/coq,maximedenes/coq,letouzey/coq-wip,ejgallego/coq,gares/coq,maximedenes/coq,silene/coq,SkySkimmer/coq,Lionel-Rieg/coq,maximedenes/coq,coq/coq,Lionel-Rieg/coq,william-lawvere/coq,Zimmi48/coq,JasonGross/coq,SkySkimmer/coq,JasonGross/coq,gares/coq,silene/coq,ppedrot/coq,JasonGross/coq,letouzey/coq-wip,Zimmi48/coq,amintimany/coq,amintimany/coq,coq/coq,SkySkimmer/coq