Add getSerial() routine that return current revision id for an oid.

BaseStorage implements this in terms of modifiedInVersion() and
load().  Other storages may (perhaps even should) implement it more
efficiently.

src/ZODB/BaseStorage.py

@@ -86,7 +86,7 @@
 """
 # Do this portably in the face of checking out with -kv
 import string
-__version__ = string.split('$Revision: 1.14 $')[-2:][0]
+__version__ = string.split('$Revision: 1.15 $')[-2:][0]
 
 import ThreadLock, bpthread
 import time, UndoLogCompatible
@@ -253,6 +253,15 @@ class BaseStorage(UndoLogCompatible.UndoLogCompatible):
 
     def pack(self, t, referencesf): pass
 
+    def getSerial(self, oid):
+        self._lock_acquire()
+        try:
+            v = self.modifiedInVersion(oid)
+            pickledata, serial = self.load(oid, v)
+            return serial
+        finally:
+            self._lock_release()
+
     def loadSerial(self, oid, serial):
         raise POSException.Unsupported, (
             "Retrieval of historical revisions is not supported")