MINOR: spoe: Don't queue a SPOE context if nothing is sent

MINOR: spoe: Don't queue a SPOE context if nothing is sent

When some messages must be sent to an agent, the SPOE context of the stream is
queued to be handled by an SPOE applet. If there is no available applet, a new
one is created, thus opening a connection with the agent.

Since the support of ACLs on messages, some processing can now be discarded. So,
to avoid opening a connection for nothing, the SPOE context is now queued after
the messages encoding.