virtcontainers: store: Add a ItemLock API

The ItemLock API allows for taking shared and exclusive locks on all
items.
For virtcontainers, this is specialized into taking locks on the Lock
item, and will be used for sandbox locking.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

virtcontainers/store/backend.go

@@ -53,4 +53,6 @@ type backend interface {
 	// The caller gets an item URL back and handles it directly,
 	// outside of the top level Store API.
 	raw(id string) (string, error)
+	lock(item Item, exclusive bool) (string, error)
+	unlock(item Item, token string) error
 }