Currently when docker exits ungracefully it may leave
dangling sandboxes which may hold onto precious network
resources. Added checkpoint state for sandboxes which
on boot up will be used to clean up the sandboxes and
network resources.
On bootup the remaining dangling state in the checkpoint
are read and cleaned up before accepting any new
network allocation requests.
Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>