Drop the per-task timing stats, give a summary only (#5897)

We were keeping around the Task struct (216 bytes) for every worker we
ever created, even though we only keep a maximum of 6 workers per
Capability.  These Task structs accumulate and cause a space leak in
programs that do lots of safe FFI calls; this patch frees the Task
struct as soon as a worker exits.

One reason we were keeping the Task structs around is because we print
out per-Task timing stats in +RTS -s, but that isn't terribly useful.
What is sometimes useful is knowing how *many* Tasks there were.  So
now I'm printing a single-line summary, this is for the program in

  TASKS: 2001 (1 bound, 31 peak workers (2000 total), using -N1)

So although we created 2k tasks overall, there were only 31 workers
active at any one time (which is exactly what we expect: the program
makes 30 safe FFI calls concurrently).

This also gives an indication of how many capabilities were being
used, which is handy if you use +RTS -N without an explicit number.

MERGED from commit 085c7fe5