MINOR: activity: collect time spent waiting on a lock for each task

MINOR: activity: collect time spent waiting on a lock for each task

When DEBUG_THREAD > 0, and if task profiling is enabled, then each
locking attempt will measure the time it takes to obtain the lock, then
add that time to a thread_ctx accumulator that the scheduler will then
retrieve to update the current task's sched_activity entry. The value
will then appear avearaged over the number of calls in the lkw_avg column
of "show profiling tasks", such as below:

  Tasks activity over 48.298 sec till 0.000 sec ago:
    function                      calls   cpu_tot   cpu_avg   lkw_avg   lat_avg
    h1_io_cb                    3200170   26.81s    8.377us      -      32.73us <- sock_conn_iocb@src/sock.c:1099 tasklet_wakeup
    sc_conn_io_cb               1657841   1.645s    992.0ns      -      853.0us <- sc_app_chk_rcv_conn@src/stconn.c:844 tasklet_wakeup
    process_stream              1600450   49.16s    30.71us   1.936us   1.392ms <- sc_notify@src/stconn.c:1206 task_wakeup
    process_stream              1600321   7.770m    291.3us   209.1us   901.6us <- stream_new@src/stream.c:585 task_wakeup
    sc_conn_io_cb               1599928   7.975s    4.984us      -      65.77us <- h1_wake_stream_for_recv@src/mux_h1.c:3633 tasklet_wakeup
    task_process_applet          997609   46.37s    46.48us   16.80us   113.0us <- sc_app_chk_snd_applet@src/stconn.c:1043 appctx_wakeup
    process_table_expire         922074   48.79s    52.92us   7.275us   181.1us <- run_tasks_from_lists@src/task.c:670 task_queue
    stktable_add_pend_updates    705423   1.511s    2.142us      -      56.81us <- stktable_add_pend_updates@src/stick_table.c:869 tasklet_wakeup
    task_process_applet          683511   34.75s    50.84us   18.37us   153.3us <- __process_running_peer_sync@src/peers.c:3579 appctx_wakeup
    h1_io_cb                     535395   198.1ms   370.0ns   72.00ns   930.4us <- h1_takeover@src/mux_h1.c:5659 tasklet_wakeup

It now makes it pretty obvious which tasks (hence call chains) spend their
time waiting on a lock and for what share of their execution time.