This adds some basic ability to join a node to an existing cluster. It
uses a rpc layer to initiate a join request to an existing memeber. The
response indicates whether the joining node should take part in the raft
cluster and who it's peers should be. If raft should not be started, the
peers are the addresses of the current raft members that it should delegate
consensus operations.
To keep the meta store implementation agnostic of whether it's running
a local raft or not, a consensusStrategy type was also added.
Jason Wilder
Philip O'Toole
Ben Johnson
Cory LaNou