kverserver: store rebalancer pushes leases back to draining nodes

[knz]

This is the underlying cause of issues #53760 #53725 #54088 #53811

When a node starts draining, it starts transferring its leases away. The drain process does this until there's no lease left to transfer.

However in the meantime the store rebalancer sees the QPS on the other stores, which are taking on the draining node's load, increase. So it then selects the now-draining node as a good candidate to transfer leases to, because it appears to be low on QPS (because it's draining)

So the leases are transferred back to the draining node.

So the graceful drain never completes.
And the node eventually is killed with active leases.
Which disrupts performance.

[knz]

The allocator does it too.