Report status of chain tips

This makes the output a bit more intuitive when running on a synchronizing headers-first client:
```
[
    {
        "height" : 310985,
        "hash" : "000000004b9d96cf156f9e74acdf03647a2f8665c5d98693ef5a9130efcc0106",
        "branchlen" : 182402,
        "status" : "headers-only"
    },
    {
        "height" : 128583,
        "hash" : "00000000119df3d17af48af07480d3d5a5966cfad28f40b08a79b55b02fd8df2",
        "branchlen" : 0,
        "status" : "active"
    }
]
```