Support Cluster Mesh Sync

[riebecj]

Is your feature request related to a problem? Please describe.
There's no problem, per-say, I would just like to have a mesh (many-to-many) synchronization of changes between nodes. When I have 2 nodes and each node adds the other as an "Environment" or "Node", and I sync a site from one node to the other, the other node's sync settings get disabled. I have a keepalived VIP configured between the nodes, and if one goes down and the other becomes the main entry point, and changes are made, unless I re-select the nodes again, the changes don't get synced.

Describe the solution you'd like
It honestly works "as-is" except for the nodes that receive the pub/sub message disable their own sync settings. If possible, just don't disable local node sync settings automatically.

Describe alternatives you've considered
The alternative is to just remember to select the nodes before saving, which isn't ideal but it's also not the end of the world.

Additional context
N/A