Make the cache thread safe again. :/

There are some cases where we want to touch the cache outside of the
I/O thread. Including:

- ClientStorage wants to to invalidata cache entries if it gets a
  conflict error in voting.

- loadBefore can probably be optimized by checking the cache in the
  client thread. (This will be safe for loadBefore, because the intent
  in practice is never to load current data.

src/ZEO/interfaces.py

@@ -24,8 +24,7 @@ class StaleCache(object):
 class IClientCache(zope.interface.Interface):
     """Client cache interface.
 
-    Note that caches need not be thread safe, fpr the most part,
-    except for getLastTid, which may be called from multiple threads.
+    Note that caches need to be thread safe.
     """
 
     def close():