raft: Test initial load of a snapshot
raft: Test initial load of a snapshot

There was test coverage in place for receiving a snapshot over RPC, but
not for loading a previously stored snapshot from disk. This extends
TestRaftSnapshotRestart to do a second restart, which involves loading
the snapshot from disk.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

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