Fix flaky maxSubmitRetries test

This is a slightly speculative fix for a test that fails intermittently
on `sharedb-mongo`. I believe these intermittent failures are due to a
race condition in a concurrency test.

The test works by attempting to fire two commits off at the same time,
and hoping that one of them is committed just before the other, so that
a `SubmitRequest.retry` is triggered whilst the `maxSubmitRetries` is
set to `0`, resulting in an error that is expected.

However, I believe it's possible for these commits to (in some cases)
happen sequentially rather than concurrently, and fail to error.

This change attempts to force them into this retry condition by:

  - Catching both ops in the `commit` middleware, _just_ before they're
    about to be committed (and hit a `retry` if applicable)
  - Waiting until both ops have reached this state
  - Triggering the first op's `commit`
  - Then in the callback of that op, triggering the second op's `commit`
  - The second op should now find that the first op has beaten it to
    committing, and trigger a `retry`

Author: Alec Gibson

test/client/submit.js

@@ -509,25 +509,33 @@ describe('client submit', function() {
   });
 
   it('submits fail above the backend.maxSubmitRetries threshold', function(done) {
+    var backend = this.backend;
     this.backend.maxSubmitRetries = 0;
     var doc = this.backend.connect().get('dogs', 'fido');
     var doc2 = this.backend.connect().get('dogs', 'fido');
     doc.create({age: 3}, function(err) {
       if (err) return done(err);
       doc2.fetch(function(err) {
         if (err) return done(err);
-        var count = 0;
-        var cb = function(err) {
-          count++;
-          if (count === 1) {
-            if (err) return done(err);
-          } else {
-            expect(err).ok();
-            done();
+        var docCallback;
+        var doc2Callback;
+        backend.use('commit', function (request, callback) {
+          if (request.op.op[0].na === 2) docCallback = callback;
+          if (request.op.op[0].na === 7) doc2Callback = callback;
+
+          if (docCallback && doc2Callback) {
+            docCallback();
           }
-        };
-        doc.submitOp({p: ['age'], na: 2}, cb);
-        doc2.submitOp({p: ['age'], na: 7}, cb);
+        });
+
+        doc.submitOp({p: ['age'], na: 2}, function (error) {
+          if (error) return done(error);
+          doc2Callback();
+        });
+        doc2.submitOp({p: ['age'], na: 7}, function (error) {
+          expect(error).ok();
+          done();
+        });
       });
     });
   });