incorrect batch group size computation for write throttling

Summary:
When a write batch can't join a batch group due to the total
size of the contained batches, the write controller's GetDelay is passed
a size value that includes the rejected batch.

Test Plan: make check

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D50343

db/write_thread.cc

@@ -120,12 +120,13 @@ size_t WriteThread::EnterAsBatchGroupLeader(
       break;
     }
 
-    size += WriteBatchInternal::ByteSize(w->batch);
-    if (size > max_size) {
+    auto batch_size = WriteBatchInternal::ByteSize(w->batch);
+    if (size + batch_size > max_size) {
       // Do not make batch too big
       break;
     }
 
+    size += batch_size;
     write_batch_group->push_back(w->batch);
     w->in_batch_group = true;
     *last_writer = w;