`bucket_send()` and `bucket_recv()` should check `rebalancer_transfering_buckets` before touching it

`bucket_send(bid)` sets `M.rebalancer_transfering_buckets[bid] = true` without checking if it was already true.
1) Need to check it and fail immediately if the bucket is in transfer already.
2) See if the current behaviour can lead to any bugs. Cover them with tests.

Same with `bucket_recv()`.