directories: make the IV for the writecaps in directory entries be computed from the secure hash of the writecap itself

This makes encoding of directory entries deterministic, and it is also a tad faster on Macbook Pro than getting a random IV with os.urandom(16).

src/allmydata/dirnode.py

@@ -195,7 +195,7 @@ class NewDirectoryNode:
 
     def _encrypt_rwcap(self, rwcap):
         assert isinstance(rwcap, str)
-        IV = os.urandom(16)
+        IV = hashutil.mutable_rwcap_iv_hash(self._node.get_writekey())
         key = hashutil.mutable_rwcap_key_hash(IV, self._node.get_writekey())
         cryptor = AES(key)
         crypttext = cryptor.process(rwcap)