Rework push reservation w/ datastore

- At Handle creation, first check if an instance of the
  the respective object is already present in the datastore.
- Handle sequence must be saved only if commit
  to datastore is succesfull
- Caller (ipam) needs to manage the retry

Signed-off-by: Alessandro Boch <aboch@docker.com>

bitseq/store.go

@@ -19,7 +19,7 @@ func (h *Handle) KeyPrefix() []string {
 	return []string{h.App}
 }
 
-// Value marshala the data to be stored in the KV store
+// Value marshals the data to be stored in the KV store
 func (h *Handle) Value() []byte {
 	h.Lock()
 	defer h.Unlock()