feat(load-balance): update selection strategy for max load disk

[hycdong]

In cluster load balance, we will try to migrate partition from max_node who has the max replica count to min_node, then we would like to pick up a max_load_disk who has the max replica count in max_node.
The old pickup strategy, if different disks hold same replica count, it will chosse the first one.

max_node:                   |               min_node:
ssd1: 2.1, 2.2              |                2.1, 2.2, 2.5
ssd2: 2.3, 2.4              |
ssd3: 2.6                   |
...

Just as the example shows, ssd1 will be choosen. However, 2.1 and 2.2 have already exsited in min_node, so there won't be any partition to be moved.

This pr fixs the problem above. We use get_max_load_disk_set to get a disk set. ssd1 and ssd2 will be included in the set. Then pick_up_move will choose one of them randomly.