MAJOR: stream: don't initialize the stream anymore in stream_accept

MAJOR: stream: don't initialize the stream anymore in stream_accept

The function now only initializes a session, calls the tcp req connection
rules, and calls stream_complete() to finish initialization. If a handshake
is needed, it is done without allocating the stream at all.

Temporarily, in order to limit the amount of changes, the task allocated
is put into sess->task, and it is used by the connection for the handshake
or is offered to the stream. At this point we set the relation between
sess/task/conn this way :

        orig -- sess  <-- context
         |       ^ +- task -+  |
         v       |          v  |
        conn -- owner       task

The task must not remain in the session and ultimately it is planned to
remove this task pointer from the session because it can be found by
having conn->owner = task, and looping back from sess to conn, and to
find the session from the connection via the task.