Connection.new_oid delegates to its storage, not the DB.

Fixes #139

CHANGES.rst

@@ -2,6 +2,13 @@
  Change History
 ================
 
+5.1.2 (unreleased)
+==================
+
+- ``Connection.new_oid`` delegates to its storage, not the DB. This is
+  helpful for improving concurrency in MVCC storages like RelStorage.
+  See `issue 139 <https://github.com/zopefoundation/ZODB/issues/139`_.
+
 5.1.1 (2016-11-18)
 ==================
 

src/ZODB/Connection.py

@@ -126,7 +126,6 @@ class Connection(ExportImport, object):
             storage = storage.new_instance()
 
         self._normal_storage = self._storage = storage
-        self.new_oid = db.new_oid
         self._savepoint_storage = None
 
         # Do we need to join a txn manager?
@@ -200,6 +199,9 @@ class Connection(ExportImport, object):
 
         self._reader = ObjectReader(self, self._cache, self._db.classFactory)
 
+    def new_oid(self):
+        return self._storage.new_oid()
+
     def add(self, obj):
         """Add a new object 'obj' to the database and assign it an oid."""
         if self.opened is None: