Use column keys in Bigtable

[tomwilkie]

After chatting to @mbrukman, it appears BigTable does indeed support column keys with the semantics we need:

https://cloud.google.com/bigtable/docs/go/reference#ColumnRangeFilter

This should improve query performance as there will be much less repetition of the row key. Also, whilst we're at it, we should hash the row key to improve load balancing.

This will need to be done with a --from flag to maintain backwards compatibility.