Fix getuidvalidity crash (UIDVALIDITY returning None)

Rename getuidvalidity -> get_uidvalidity and cache the IMAP result.

1) Start modernizing our function names using more underscores
2) IMAPs implementation of get_uidvalidity was removing the UIDVALIDITY result
   from the imaplib2 result stack, so subsequent calls would return "None".
   As various functions can invoke this, this led to some errors that we
   avoid by caching the current UIDVALIDITY value in the Folder instance.

There are more simplifications and improvements to be made.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/IMAP.py

@@ -65,17 +65,23 @@ class IMAPFolder(BaseFolder):
     def getcopyinstancelimit(self):
         return 'MSGCOPY_' + self.repository.getname()
 
-    def getuidvalidity(self):
+    def get_uidvalidity(self):
+        """Retrieve the current connections UIDVALIDITY value
+
+        UIDVALIDITY value will be cached on the first call.
+        :returns: The UIDVALIDITY as (long) number."""
+        if hasattr(self, '_uidvalidity'):
+            # use cached value if existing
+            return self._uidvalidity
         imapobj = self.imapserver.acquireconnection()
         try:
-            # SELECT receives UIDVALIDITY response
+            # SELECT (if not already done) and get current UIDVALIDITY
             self.selectro(imapobj)
-            # note: we would want to use .response() here but that
-            # often seems to return [None], even though we have
-            # data. TODO
-            uidval = imapobj._get_untagged_response('UIDVALIDITY')
-            assert uidval != [None], "response('UIDVALIDITY') returned [None]!"
-            return long(uidval[-1])
+            typ, uidval = imapobj.response('UIDVALIDITY')
+            assert uidval != [None] and uidval != None, \
+                "response('UIDVALIDITY') returned [None]!"
+            self._uidvalidity = long(uidval[-1])
+            return self._uidvalidity
         finally:
             self.imapserver.releaseconnection(imapobj)