NewResourceMap initializes an empty ReverseMap.

CachedStatus stores the status and condition for one resource.

ResourceMap maintains the following maps: 1) resToResgroups maps a resource to all the resource groups including it 2) resgroupToResources maps a resource group to its resource set 3) resToStatus maps a resource to its cached status 4) gkToResources maps a GroupKind to its resource set During the reconciliation of a RG in the root controller, the updates to these two maps should be atomic.