Add freeScheduler/freeTaskManager and call it later than exitScheduler

Add freeScheduler/freeTaskManager and call it later than exitScheduler
We were freeing the tasks in exitScheduler (stopTaskManager) before
exitStorage (stat_exit), but the latter needs to walk down the list
printing stats. Resulted in segfaults with commands like
    ghc -v0 -e main q.hs -H32m -H32m +RTS -Sstderr
(where q.hs is trivial), but very sensitive to exact commandline and
libc version or something.