Fix crash when client submits changeset based on too-old revision

We had a problem with the server running out of stack space if a client
submitted a changeset based on a revision more than about 1000 revs old.
(944 was our cutoff but yours may vary). This happened in the wild with
about 30 people editing via flaky wifi. A disconnected client would try
to submit a fairly old changeset when reconnecting, and a few minutes
was enough for 30 people to generate that many revs.

The stack kept growing because pad.getRevisionChangeset was being answered
from the cache, so no I/O interrupted the callback chain. (This was seen with
mysql, I don't know about other backends.)

This patch forces a nextTick every 200 revisions to solve this problem.

src/node/handler/PadMessageHandler.js

@@ -526,7 +526,11 @@ function handleUserChanges(client, message)
             if(ERR(err, callback)) return;
             
             changeset = Changeset.follow(c, changeset, false, apool);
-            callback(null);
+            if ((r - baseRev) % 200 == 0) { // don't let the stack get too deep
+              async.nextTick(callback);
+            } else {
+              callback(null);
+            }
           });
         },
         //use the callback of the series function