MAJOR: session: only allocate buffers when needed

MAJOR: session: only allocate buffers when needed

A session doesn't need buffers all the time, especially when they're
empty. With this patch, we don't allocate buffers anymore when the
session is initialized, we only allocate them in two cases :

  - during process_session()
  - during I/O operations

During process_session(), we try hard to allocate both buffers at once
so that we know for sure that a started operation can complete. Indeed,
a previous version of this patch used to allocate one buffer at a time,
but it can result in a deadlock when all buffers are allocated for
requests for example, and there's no buffer left to emit error responses.
Here, if any of the buffers cannot be allocated, the whole operation is
cancelled and the session is added at the tail of the buffer wait queue.

At the end of process_session(), a call to session_release_buffers() is
done so that we can offer unused buffers to other sessions waiting for
them.

For I/O operations, we only need to allocate a buffer on the Rx path.
For this, we only allocate a single buffer but ensure that at least two
are available to avoid the deadlock situation. In case buffers are not
available, SI_FL_WAIT_ROOM is set on the stream interface and the session
is queued. Unused buffers resulting either from a successful send() or
from an unused read buffer are offered to pending sessions during the
->wake() callback.