Xid -> uid hashing function

[mrjn (Manish R Jain)]

Hey @matthiasg,

We use go-farm.Fingerprint64. See here:
https://github.com/dgraph-io/dgraph/blob/master/rdf/parse.go#L42

The package is here:
GitHub - dgryski/go-farm: go-farm: a pure-Go farmhash implementation, which is an implementation of:

GitHub - google/farmhash: Automatically exported from code.google.com/p/farmhash