Add RoundRobin URL selection per remote

Introduce a `urlDistributionStrategy` config flag on remote sections.
When set to `roundRobin`, each replication push selects one URL from the
configured list in round-robin order rather than pushing to all URLs
simultaneously.

This is particularly useful in high-availability setups where multiple
replica hosts share a single NFS backend. Pushing to all URLs at once
would result in redundant writes to the same underlying storage.
Round-robin distributes push load evenly across hosts while ensuring
each push is executed exactly once.

Change-Id: Id67cddcf9e7f7a6f47f259eb78b0f53f0977f536