Provide support for multiple independent chains

[sue.sml2006@…]

Currently it seems TracTicketChainedFields only supports one field at the top of the chain. It would be nice if we could have multiple independent chains. Perhaps there is a way to hack it, but the following doesn't work today even though it's valid json. Only the second set of chained fields is considered

{
    "tcf_Component": {
        "component1":{
            "tcf_subcomponent":{
                "Sub1":{}
            }
        },
        "component2":{
            "tcf_subcomponent":{
                "Sub2":{}
            }
        },
        "testing":{
            "tcf_Subcomponent":{
                "Sub2":{}
            }
        }
    },
    "tcf_foundinphase": {
        "Unknown":{
            "tcf_feature":{
                "Unknown":{}
            }
        },
        "System Integration":{
            "tcf_feature":{
                "Feature2":{}
            }
        },
        "Unit Test":{
            "tcf_feature":{
                "Feature1":{}
            }
        },
        "System Test":{
            "tcf_feature":{
                "Feature3":{}
            }
        },
        "Customer":{
            "tcf_feature":{
                "Feature1":{}
            }
        }
    }           
}