make alternative node selection more deterministic when hostname in remote cluster ref is unusable

Beginning in 4.6.2, when xdcr detects that the hostname in remote cluster ref is unusable, i.e., it is no longer in the target cluster, it replaces hostname with a random working node in target cluster. If there are many source nodes in source cluster, each source node may pick a different replacement node. This would create a large number of different update request on metakv, and result in an pingpong behavior due to the issue in MB-25004.

One way to avoid the pingpong behavior is for xdcr to pick alternative target node in a more deterministic manner, so that all source nodes would pick the same alternative target node.