BUG/MINOR: mux-h2: do not wake up blocked streams before the mux is ready

BUG/MINOR: mux-h2: do not wake up blocked streams before the mux is ready

In h2_send() we used to scan pending streams and wake them up when it's
possible to send, without considering the connection's state. Thus caused
some excess failed calls to h2_snd_buf() during the preface on backend
connections :

[01|h2|4|mux_h2.c:3562] h2_wake(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:3475] h2_process(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:3326] h2_send(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:3152] h2_process_mux(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:1508] h2c_bck_send_preface(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:1379] h2c_send_settings(): entering : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:1464] h2c_send_settings(): leaving : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:1543] h2c_bck_send_preface(): leaving : h2c=0x7f1430032ed0(B,PRF)
[01|h2|4|mux_h2.c:3241] h2_process_mux(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|3|mux_h2.c:3384] sent data : h2c=0x7f1430032ed0(B,STG)
  >>> streams woken up here
[01|h2|4|mux_h2.c:3428] h2_send(): waking up pending stream : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3435] h2_send(): leaving with everything sent : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3326] h2_send(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3152] h2_process_mux(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3241] h2_process_mux(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3435] h2_send(): leaving with everything sent : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3552] h2_process(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3564] h2_wake(): leaving
  >>> I/O callback was already scheduled and called despite having nothing left to do
[01|h2|4|mux_h2.c:3454] h2_io_cb(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3326] h2_send(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3152] h2_process_mux(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3241] h2_process_mux(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3435] h2_send(): leaving with everything sent : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:3463] h2_io_cb(): leaving
  >>> stream tries and fails again here!
[01|h2|4|mux_h2.c:5568] h2_snd_buf(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5587] h2_snd_buf(): connection not ready, leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5398] h2_subscribe(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5408] h2_subscribe(): subscribe(send) : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5422] h2_subscribe(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5475] h2_rcv_buf(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5535] h2_rcv_buf(): leaving : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5398] h2_subscribe(): entering : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5400] h2_subscribe(): subscribe(recv) : h2c=0x7f1430032ed0(B,STG)
[01|h2|4|mux_h2.c:5422] h2_subscribe(): leaving : h2c=0x7f1430032ed0(B,STG)

This can happen when sending the preface, the settings, and the settings
ACK. Let's simply condition the wake up on st0 >= FRAME_H as is done at
other places.