doc: add note about discovery proxy
Merge pull request #1320 from jonboulle/proxy_doc

doc: add note about discovery proxy
main: correct peer-addr reference
Merge pull request #1321 from jonboulle/peers

main: correct peer-addr reference
etcdserver: fix data race in retrieving self stats
etcdserver: fix race and improve stats interfaces
etcdserver: copy stats instead of marshaling with lock
etcdserver/stats: log any marshaling error