core (memory/persistent): added aset method in LazyPersistentBinaryDict:

this method does the same as `force` but is there to be consistent with PersistentDict.
`force` should be removed in the future.

sat/memory/persistent.py

@@ -27,7 +27,7 @@ class MemoryNotInitializedError(Exception):
     pass
 
 
-class PersistentDict(object):
+class PersistentDict:
     r"""A dictionary which save persistently each value assigned
 
     /!\ be careful, each assignment means a database write
@@ -250,6 +250,13 @@ class LazyPersistentBinaryDict(PersistentBinaryDict):
         d.addErrback(self._defaultOrException, default=default)
         return d
 
+    def aset(self, key, value):
+        """Async set, return a Deferred fired when value is actually stored"""
+        # FIXME: redundant with force, force must be removed
+        # XXX: similar as PersistentDict.aset, but doesn't use cache
+        return self.storage.setPrivateValue(self.namespace, key, value,
+                                            self.binary, self.profile)
+
     def setdefault(self, key, default):
         raise NotImplementedError