Fix ingestion path replication

[joe-elliott]

We may have gotten too excited during the great butchering of Loki and removed more than was intended. In the distributor there is a method routeRequest that breaks a PushRequest up by traceid and forwards it to the appropriate ingesters based on tokens/replication factor. After this the distributor sends

Unfortunately it lost the ability to check for concensus on write. Readd this. Look at the replicationSet.Do() method from Cortex b/c I think it does this already.

[joe-elliott]

This is basically covered by #119