Checkpoint: start with seq=1 instead of seq=0

Since the seq behaves in many senses like an id, it should meet
the usual expectation people have about ids. Using only truthy values
is one of them.

common/models/checkpoint.js

@@ -41,7 +41,7 @@ module.exports = function(Checkpoint) {
       if (checkpoint) {
         cb(null, checkpoint.seq);
       } else {
-        Checkpoint.create({seq: 0}, function(err, checkpoint) {
+        Checkpoint.create({ seq: 1 }, function(err, checkpoint) {
           if (err) return cb(err);
           cb(null, checkpoint.seq);
         });

test/checkpoint.test.js

@@ -14,7 +14,7 @@ describe('Checkpoint', function() {
         function(next) {
           Checkpoint.current(function(err, seq) {
             if (err) next(err);
-            expect(seq).to.equal(2);
+            expect(seq).to.equal(3);
             next();
           });
         }