Homestyx hydra

R1:51073b972ece

Try to route cluster writes to nodes which won't need to synchronize first

Summary:
Ref T13109. Ref T13202. See PHI905. See PHI889. When we receive a write to a repository cluster, we currently send it to a random writable node.

Instead, we can prefer:

- the node currently holding the write lock; or
- any node which is already up to date.

These should simply be better nodes to take writes in all cases. The write lock is global for the repository, so there's no scaling benefit to spreading writes…
Repository: R1 hydra
Commit Date: Oct 17 2018