Adaptive parallelism in the vbucket move

It takes a couple of minutes (I didn't measure that accurate, but I would say ~10) on my machine when I try to add the second node to my cluster with 1 bucket and NO items. Both of these nodes is running on the same server, and there is plenty of CPU left on the box to speed up the transfer. Given that ns_server knows the total amount of data kept in the bucket, it could move more buckets in parallel (in this case we've got 0 items, so we could most likely move all of the vbuckets it wants in parallel). As the item count and memory usage grows it should reduce the parallelism.