HistoricalStorageAdapter forwards release() to its instance

Fixes #78.

Also adjust the base wrapper's __getattr__ to be faster (500 vs 711ns
in a benchmark of the common case) and behave the same on Python 2 vs
Python 3 with regards to exceptions, should a method be implemented as
an descriptor. In practice this probably makes no difference
whatsoever, it's just the same pattern used for forwarding
release.

CHANGES.rst

@@ -9,6 +9,8 @@
 
 - Drop support for Python 3.3.
 
+- The ``HistoricalStorageAdapter`` forwards the ``release`` method to
+  its base instance. See `issue 78 <https://github.com/zopefoundation/ZODB/issues/788>`_.
 
 5.2.4 (2017-05-17)
 ==================

src/ZODB/mvccadapter.py

@@ -27,8 +27,11 @@ def __init__(self, storage):
 
     def __getattr__(self, name):
         if name in self._copy_methods:
-            if hasattr(self._storage, name):
+            try:
                 m = getattr(self._storage, name)
+            except AttributeError:
+                pass
+            else:
                 setattr(self, name, m)
                 return m
 
@@ -204,7 +207,12 @@ def supportsUndo(self):
         return False
 
     def release(self):
-        pass
+        try:
+            release = self._storage.release
+        except AttributeError:
+            pass
+        else:
+            release()
 
     close = release