Use "with lock" pattern

While looking at the code to investigate if an why we sometimes don't
seem to honor the write lock, I made it use the more modern "with lock:"
pattern.

Still have not found out how we could ever be using 2 instances of the
LocalStatusFolder for the same folder.

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

offlineimap/folder/LocalStatus.py

@@ -85,8 +85,7 @@ class LocalStatusFolder(BaseFolder):
         file.close()
 
     def save(self):
-        self.savelock.acquire()
-        try:
+        with self.savelock:
             file = open(self.filename + ".tmp", "wt")
             file.write(magicline + "\n")
             for msg in self.messagelist.values():
@@ -104,9 +103,6 @@ class LocalStatusFolder(BaseFolder):
                 os.fsync(fd)
                 os.close(fd)
 
-        finally:
-            self.savelock.release()
-
     def getmessagelist(self):
         return self.messagelist