rpcproxy is refactored into serverlist which prioritizes good servers
over servers in a remote DC or who have had a failure.
Registration, heartbeating, and alloc status updating will retry faster
when new servers are discovered.
Consul discovery will be retried more quickly when no servers are
available (eg on startup or an outage).
When an agent is running a server, the list of servers includes the
Raft peers. When the agent is running a client (which is always the
case?), include a list of the servers found in the Client's RpcProxy.
Dedupe and provide a unique list back to the caller.
