Bring back parallel position pickliing

[odersky]

http://dotty-bench.epfl.ch/9633/

shows that parallel position pickling led to significant speedups. (the last commit in those sequences is the first where parallel position pickling was disabled again and is otherwise virtually identical to the previous lower points of the the PR 9633.

So, we should try to bring it back.

[liufengyun]

Fixed in #9650