concurrency.distributed: replace remote-thread connection slot with assoc
concurrency.distributed: replace remote-thread connection slot with assoc

The new global assoc will match remote-thread instances with their
connection instances, if any. The slot removal will reduce the burden of
sending the remote-thread instances over the socket.

mrjbq7/factor,factor/factor,factor/factor,tgunr/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,AlexIljin/factor,bpollack/factor,bpollack/factor,tgunr/factor,bpollack/factor,AlexIljin/factor,bpollack/factor,tgunr/factor,AlexIljin/factor,AlexIljin/factor,AlexIljin/factor,factor/factor,bpollack/factor,mrjbq7/factor,AlexIljin/factor,tgunr/factor,factor/factor,tgunr/factor,factor/factor,bpollack/factor,tgunr/factor,factor/factor,mrjbq7/factor,mrjbq7/factor,mrjbq7/factor