CorePinnedPool: fix PinnedProcess not picklable

This requires taking the PinnedProcess class definition to the module's top-level, which means yet another hackish thing: assuming the ctx.Process is a ForkServerProcess and asserting over that.

This brings another problem : the core queue was not passed to the child processes, hence unable to pick their core. I had to pass it hackishly, pretty much mimicking what is done for the error-report pipe.