MEDIUM: mux-h1: do not blindly wake up the tasklet at end of request anymore

MEDIUM: mux-h1: do not blindly wake up the tasklet at end of request anymore

Since commit "MEDIUM: connection: make the subscribe() call able to wakeup
if ready" we have the guarantee that the tasklet will be woken up if
subscribing to a connection for an even that's ready. Since we have too
many tasklet_wakeup() calls in mux-h1, let's now use this property to
improve the situation a bit.

With this change, no syscall count changed, however the number of useless
calls to some functions significantly went down. Here are the differences
for the test below (100k req), in number of calls per request :

  $ ./h1load -n 100000 -t 4 -c 1000 -T 20 -F 127.0.0.1:8001/?s=1k/t=20

                           before   after  change   note
  tasklet_wakeup:           3        1      -66%
  h1_io_cb:                 4        3      -25%
  h1_send:                  6.7      5.4    -19%
  h1_wake:                  0.73     0.44   -39%
  h1_process:               4.7      3.4    -27%
  h1_wake_stream_for_send:  6.7      5.5    -18%
  si_cs_process             3.7      3.4     -7.8%
  conn_fd_handler           2.7      2.4    -10%
  raw_sock_to_buf:          4        2      -50%
  pool_free:                4        2      -50%    from failed rx calls

Note that the situation could be further improved by having muxes lazily
subscribe to Rx events in case the FD is already being polled. However
this requires deeper changes to implement a LAZY_RECV subscribe mode,
and to replace the FD's active bit by 3 states representing the desired
action to perform on the FD during the update, among NONE (no need to
change), POLL (can't proceed without), and STOP (buffer full). This
would only impact Rx since on Tx we know what we have to send. The
savings to expect from this might be more visible with splicing and/or
when dealing with many connections having long think times.