MINOR: activity: also report collected tasks stats in "show profiling"

MINOR: activity: also report collected tasks stats in "show profiling"

"show profiling" will now dump the stats collected by the scheduler if
profiling was previously enabled. This will immediately make it obvious
what functions are responsible for others' high latencies or which ones
are suffering from others, and should help spot issues like undesired
wakeups.

Example:

Per-task CPU profiling              : on      # set profiling tasks {on|auto|off}
Tasks activity:
  function                      calls   cpu_tot   cpu_avg   lat_tot   lat_avg
  si_cs_io_cb                 5569479   23.37s    4.196us      -         -
  h1_io_cb                    5558654   13.60s    2.446us      -         -
  process_stream               250841   1.476s    5.882us   3.499s    13.95us
  main+0x9e670                    198      -         -      5.526ms   27.91us
  task_run_applet                  17   1.509ms   88.77us   205.8us   12.11us
  srv_cleanup_idle_connections     12   44.51us   3.708us   25.71us   2.142us
  main+0x158c80                     9   48.72us   5.413us      -         -
  srv_cleanup_toremove_connections  5   165.1us   33.02us   123.6us   24.72us