Detect bug 6252 (unexpected sendme cell)

I only check on circuits, not streams, since bloating your stream
window past the initial circuit window can't help you much.

Also, I compare to CIRCWINDOW_START_MAX so we don't have surprising
races if we lower CIRCWINDOW_START for an experiment.

changes/bug6252

@@ -0,0 +1,8 @@
+  o Security fixes:
+    - Tear down the circuit if we get an unexpected SENDME cell. Clients
+      could use this trick to make their circuits receive cells faster
+      than our flow control would have allowed, or to gum up the network,
+      or possibly to do targeted memory denial-of-service attacks on
+      entry nodes. Fixes bug 6252. Bugfix on the 54th commit on Tor --
+      from July 2002, before the release of Tor 0.0.0.
+