OPTIM: mux-h1: subscribe rather than waking up at a few other places

OPTIM: mux-h1: subscribe rather than waking up at a few other places

This is another round of conversion from a blind tasklet_wakeup() to
a more careful subscribe(). It has significantly improved the number
of function calls per HTTP request (/?s=1k/t=20) :
                        before    after
 tasklet_wakeup:          3         2
 conn_subscribe:          3         2
 h1_iocb:                 3         2
 h1_process:              3         2
 h1_parse_msg_hdrs:       4         3
 h1_rcv_buf:              5         3
 h1_send:                 5         4
 h1_subscribe:            2         1
 h1_wake_stream_for_send: 5         4
 http_wait_for_request:   2         1
 process_stream:          3         2
 si_cs_io_cb:             4         2
 si_cs_process:           3         1
 si_cs_rcv:               5         3
 si_sync_send:            2         1
 si_update_both:          2         1
 stream_int_chk_rcv_conn: 3         2
 stream_int_notify:       3         1
 stream_release_buffers:  9         4