BUG/MEDIUM: quic-be: too early SSL_SESSION initialization

BUG/MEDIUM: quic-be: too early SSL_SESSION initialization

When an SNI is set on a QUIC server line, ssl_sock_set_servername() is called
from connect_server() (backend.c). This leads some BUG_ON() to be triggered
because the CO_FL_WAIT_L6_CONN | CO_FL_SSL_WAIT_HS were not set. This must
be done into the ->init() xprt callback. This patch move the flags settings
from ->start() to ->init() callback.

Indeed, connect_server() calls these functions in this order:

   ->init(),
   ssl_sock_set_servername() # => crash if CO_FL_WAIT_L6_CONN | CO_FL_SSL_WAIT_HS not set
   ->start()

Furthermore ssl_sock_set_servername() has a side effect to reset the SSL_SESSION
object (attached to SSL object) calling SSL_set_session(), leading to crashes as follows:

 [Thread debugging using libthread_db enabled]
 Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
 Core was generated by `./haproxy -f quic_srv.cfg'.
 Program terminated with signal SIGSEGV, Segmentation fault.
 #0  tls_process_server_hello (s=0x560c259733b0, pkt=0x7fffac239f20)
     at ssl/statem/statem_clnt.c:1624
 1624            if (s->session->session_id_length > 0) {
 [Current thread is 1 (Thread 0x7fc364e53dc0 (LWP 35514))]
 (gdb) bt
 #0  tls_process_server_hello (s=0x560c259733b0, pkt=0x7fffac239f20)
     at ssl/statem/statem_clnt.c:1624
 #1  0x00007fc36540fba4 in ossl_statem_client_process_message (s=0x560c259733b0,
     pkt=0x7fffac239f20) at ssl/statem/statem_clnt.c:1042
 #2  0x00007fc36540d028 in read_state_machine (s=0x560c259733b0) at ssl/statem/statem.c:646
 #3  0x00007fc36540ca70 in state_machine (s=0x560c259733b0, server=0)
     at ssl/statem/statem.c:439
 #4  0x00007fc36540c576 in ossl_statem_connect (s=0x560c259733b0) at ssl/statem/statem.c:250
 #5  0x00007fc3653f1698 in SSL_do_handshake (s=0x560c259733b0) at ssl/ssl_lib.c:3835
 #6  0x0000560c22620327 in qc_ssl_do_hanshake (qc=qc@entry=0x560c25961f60,
     ctx=ctx@entry=0x560c25963020) at src/quic_ssl.c:863
 #7  0x0000560c226210be in qc_ssl_provide_quic_data (len=90, data=<optimized out>,
     ctx=0x560c25963020, level=ssl_encryption_initial, ncbuf=0x560c2588bb18)
     at src/quic_ssl.c:1071
 #8  qc_ssl_provide_all_quic_data (qc=qc@entry=0x560c25961f60, ctx=0x560c25963020)
     at src/quic_ssl.c:1123
 #9  0x0000560c2260ca5f in quic_conn_io_cb (t=0x560c25962f80, context=0x560c25961f60,
     state=<optimized out>) at src/quic_conn.c:791
 #10 0x0000560c228255ed in run_tasks_from_lists (budgets=<optimized out>) at src/task.c:648
 #11 0x0000560c22825f7a in process_runnable_tasks () at src/task.c:889
 #12 0x0000560c22793dc7 in run_poll_loop () at src/haproxy.c:2836
 #13 0x0000560c22794481 in run_thread_poll_loop (data=<optimized out>) at src/haproxy.c:3056
 #14 0x0000560c2259082d in main (argc=<optimized out>, argv=<optimized out>)
     at src/haproxy.c:3667

<s> is the SSL object, and <s->session> is the SSL_SESSION object.

For the client, this is the first call do SSL_do_handshake() which initializes this
SSL_SESSION object from ->init() xpt callback. Then it is reset by
ssl_sock_set_servername(), then tls_process_server_hello() TLS stack is called with
NULL value for s->session when receiving the ServerHello TLS message.

To fix this, simply move the first call to SSL_do_handshake to ->start xprt call
back (qc_xprt_start()).

No need to backport.