R1:bc6c8c0e93a7

Explicitly shuffle nodes before selecting one for cluster sync

Summary:
Depends on D19734. Ref T13202. Ref T13109. Ref T10884. See PHI905. See PHI889. We currently rank cluster nodes in three cases:

# when performing a write, we can go to any node (D19734 should make our ranking good);
# when performing a read, we can go to any node (currently random, but T10884 discusses ideas to improve our ranking);
# when performing an internal synchronization before a read or a write, we must go to an up-to…