agent: handle initial session message
agent: handle initial session message

The role of the session message has expanded greatly in the last week,
including node and networking information. Previously, it was sufficient
to drop the first message and listen for updates. Now, there is critical
bootstrapping data that must be propagated at session startup time.

This change ensures this.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

elifa/swarmkit,AliyunContainerService/swarmkit,docker/swarmkit,alyyousuf7/swarmkit,runshenzhu/swarmkit,sungwonh/swarmkit,dperny/swarmkit-1,deljuven/swarmkit,elifa/swarmkit,dperny/swarmkit-1,fate-grand-order/swarmkit,vasil-yordanov/swarmkit,aaronlehmann/swarmkit,cezarsa/swarmkit,yp-engineering/swarmkit,docker/swarmkit,Noyan759/swarmkit,jlhawn/swarmkit,alyyousuf7/swarmkit,jlhawn/swarmkit,yongtang/swarmkit,RenaudWasTaken/swarmkit,cezarsa/swarmkit,Noyan759/swarmkit,deljuven/swarmkit,cheyang/swarmkit,yongtang/swarmkit,vasil-yordanov/swarmkit,allencloud/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,RenaudWasTaken/swarmkit,yp-engineering/swarmkit,sungwonh/swarmkit,aaronlehmann/swarmkit,runshenzhu/swarmkit,fate-grand-order/swarmkit,diogomonica/swarmkit,diogomonica/swarmkit,allencloud/swarmkit