praefect: Synchronize object pools

Currently Praefect does not support object pool syncing with the ReplicateRepository RPC. Consequently, Praefect always rewrites the ReplicateObjectDeduplicationNetworkMembership request field to false thus disabling downstream object pool syncing performed by the target Gitaly nodes.

To properly support object pool syncing, Praefect must coordinate object pool replication, linking, and disconnection from within the ReplicateRepository RPC handler. This ensures that Praefect state is appropriately updated. To facilitate this, this change introduces the syncObjectPool() function which syncs object pool repositories between the source and target Gitaly nodes.