store: Use protobuf Size method for size calculation
store: Use protobuf Size method for size calculation

This is cheaper than the original approach of marshalling every object
twice. It avoids the short-term need to switch to a streaming approach
for writing out transactions.

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

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