raftpicker: make raftpicker Stop blocking
raftpicker: make raftpicker Stop blocking

Signed-off-by: Alexander Morozov <2043cda4df124b840b98ee1c4307c199042efdf7@gmail.com>

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