Better historical connection implementation

- Lower-weight instances when not using RelStorage

- Still works with new RelStorage instances.

- Doesn't make me include loadBefore in MVCCAdapterInstance objects. :)

src/ZODB/Connection.py

@@ -103,9 +103,17 @@ class Connection(ExportImport, object):
         # Multi-database support
         self.connections = {self._db.database_name: self}
 
-        storage = db._mvcc_storage.new_instance()
+        storage = db._mvcc_storage
         if before:
-            storage = HistoricalStorageAdapter(storage, before)
+            try:
+                before_instance = storage.before_instance
+            except AttributeError:
+                def before_instance(before):
+                    return HistoricalStorageAdapter(
+                        storage.new_instance(), before)
+            storage = before_instance(before)
+        else:
+            storage = storage.new_instance()
 
         self._normal_storage = self._storage = storage
         self.new_oid = db.new_oid

src/ZODB/mvccadapter.py

@@ -53,7 +53,7 @@ class MVCCAdapter(Base):
         return instance
 
     def before_instance(self, before=None):
-        return BeforeAdapterInstance(self, before)
+        return HistoricalStorageAdapter(self._storage, before)
 
     def undo_instance(self):
         return UndoAdapterInstance(self)
@@ -97,7 +97,6 @@ class MVCCAdapterInstance(Base):
     _copy_methods = Base._copy_methods + (
         'loadSerial', 'new_oid', 'tpc_vote',
         'checkCurrentSerialInTransaction', 'tpc_abort',
-        'loadBefore',
         )
 
     def __init__(self, base):