fsharp: Fix memory leak in Parallel

Partially completed tokens waiting for the some subnets to complete
were not properly cleaned up when removing tokens.

They could not complete anymore because the other subnets did not
contain the token anymore, so they would stay in the "completed"
dictionary forever.
Merge pull request #1 from GestIT/ranma42/wip/boundmem

fsharp: Fix memory leak in Parallel
Reviewed-by: cisterni