Add a namespace object interface for somewhat cleaner code when
creating and switching between network namespaces. All created
namespaces are now mounted in /var/run/netns to ensure they
have persistent inodes and paths that can be passed around
between plugin components without relying on the current namespace
being correct.
Also remove the thread-locking arguments from the ns package
per https://github.com/appc/cni/issues/183 by doing all the namespace
changes in a separate goroutine that locks/unlocks itself, instead of
the caller having to track OS thread locking.
c0d34c692f