core (memory/persistent): implemented aset and adel:

those methods are async variants of __setitem__ and __delitem__, they return a Deferred
fired when the value is actually set/removed from storage.

sat/memory/persistent.py

@@ -127,6 +127,18 @@ class PersistentDict(object):
     def get(self, key, default=None):
         return self._cache.get(key, default)
 
+    def aset(self, key, value):
+        """Async set, return a Deferred fired when value is actually stored"""
+        self._cache.__setitem__(key, value)
+        return self.storage.setPrivateValue(self.namespace, key, value,
+                                            self.binary, self.profile)
+
+    def adel(self, key):
+        """Async del, return a Deferred fired when value is actually deleted"""
+        self._cache.__delitem__(key)
+        return self.storage.delPrivateValue(
+            self.namespace, key, self.binary, self.profile)
+
     def setdefault(self, key, default):
         try:
             return self._cache[key]