etcdserver: add ServerStats and LeaderStats

This adds the remaining two stats endpoints: `/v2/stats/self`, for
various statistics on the EtcdServer, and `/v2/stats/leader`, for
statistics on a leader's followers.

By and large most of the stats code is copied across from 0.4.x, updated
where necessary to integrate with the new decoupling of raft from
transport.

This does not satisfactorily resolve the question of name vs ID. In the
old world, names were unique in the cluster and transmitted over the
wire, so they could be used safely in all statistics. In the new world,
a given EtcdServer only knows its own name, and it is instead IDs that
are communicated among the cluster members. Hence in most places here we
simply substitute a string-encoded ID in place of name, and only where
possible do we retain the actual given name of the EtcdServer.