Rename 'keep_preferred' to 'pause_if_all_down' and accept a list of nodes

Now, a partition is paused if all nodes from the pause_if_all_down list
are seen as down. If a fraction of the list is alive, the nodes in the
partition remain up.

Compared to the previous version, some of the listed nodes can be taken
down for maintenance without risking service interruption. However, this
raises the problem of listed nodes distributed in multiple partitions:
we need to handle recovery. This will be addressed in a followup commit.

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