Create a per-client session with the router. If a
SessionManager has been installed, a proxy to a
Session
object is returned to the client. Otherwise, null is returned
and only an internal session (i.e., not visible to the client)
is created.
If a session proxy is returned, it must be configured to route
through the router that created it. This will happen automatically
if the router is configured as the client's default router at the
time the session proxy is created in the client process, otherwise
the client must configure the session proxy explicitly.