bt_mesh_subnet_kr_phase_set() function

Set the Subnet's Key Refresh phase. The Key Refresh procedure is started by updating the Subnet keys through bt_mesh_subnet_update. This puts the Subnet in Key Refresh Phase 1. Once all nodes have received the new Subnet key, Key Refresh Phase 2 can be activated through this function to start transmitting with the new network key. Finally, to revoke the old key, set the Key Refresh Phase to 3. This removes the old keys from the node, and returns the Subnet back to normal single-key operation with the new key set.