MEDIUM: connection: implement passive reverse

MEDIUM: connection: implement passive reverse

Define a new method conn_reverse(). This method is used to reverse a
connection from frontend to backend or vice-versa depending on its
initial status.

For the moment, passive reverse only is implemented. This covers the
transition from frontend to backend side. The connection is detached
from its owner session which can then be freed. Then the connection is
linked to the server instance.

only for passive connection on
frontend to transfer them on the backend side. This requires to free the
connection session after detaching it from.