MINOR: activity: add a new "show tasks" command to list currently active tasks

MINOR: activity: add a new "show tasks" command to list currently active tasks

This finally adds the long-awaited solution to inspect the run queues
and figure what is eating the CPU or causing latencies. We can even see
the experienced latencies when profiling is enabled. Example on a
saturated process:

> show tasks
Running tasks: 14983 (4 threads)
  function                     places     %    lat_tot   lat_avg
  process_stream                 4948   33.0   5.840m    70.82ms
  h1_io_cb                       2535   16.9      -         -
  main+0x9e670                   2508   16.7   2.930m    70.10ms
  ssl_sock_io_cb                 2499   16.6      -         -
  si_cs_io_cb                    2493   16.6      -         -