uri: HOST_NAME_MAX is defined elsewhere.
cluster: init cluster with uri information.

This allows us to detect the cluster mode we should be running in.
This is as follows:

 1) Use replica set mode if a ?replicaSet=Name was provided.
 2) Use sharded cluster if multiple hosts but no ?replicaSet=Name
 3) Use direct mode if a single host was provided but no ?replicaSet=Name