This fixes a problem with replacing a node with tablets when
RF=N. Currently, this will fail because new tablet replica allocation
will not be able to find a viable destination, as the replacing node
is not considered a candidate. It cannot be a candidate because
replace rolls back on failure and we cannot roll back after tablets
were migrated.
The solution taken here is to not drain tablet replicas from replaced
node during topology request but leave it to happen later after the
replaced node is left and replacing node is normal.
The replacing node waits for this draining to be complete on boot
before the node is considered booted.
Fixes#17025
1c71f44e63