Mesh: customize the replication factor for individual repositories

In the initial (Bitbucket 8.0) release of Mesh the replication factor cannot be updated, and no custom replication factor can be configured for individual repositories.

Mesh by default maintains 3 replicas of each repository. For very busy repositories, or critical repositories where even higher availability is required, we'd like the ability to specify a higher replication factor to achieve better performance and resilience.