'carbon-stale' tool to help find metrics with blank/missing data

I found this need as I was trying to sanity-verify a cluster during partial outages - primarily sanity checking after running `carbon-sync` to make sure the data I expected to be backfilled, was.

There may be more work that could be done on this to make it even more useful but for now it fills that use case reasonably well (IMHO of course).