Documentation: Clarify etcd security documentation

Signed-off-by: Vincent Ambo <vincent@kivra.com>
Update libraries-and-tools.md
etcdserver: split Member into RaftAttributes and Attributes

The split helps to save them in different key paths, because they have
distinct life cycle on update.
Update clients-matrix.md
Merge pull request #1275 from RichardHightower/patch-1

Update libraries-and-tools.md
Merge pull request #1280 from RichardHightower/patch-2

Update clients-matrix.md
etcdserver/wal: record info at the head of WAL file
Merge pull request #1236 from unihorn/153

wal: record node id at the head of WAL file
Merge pull request #1234 from unihorn/152

etcdserver: save PeerURLs and Others into two different keys
*: change all ID to uint64
Merge pull request #1289 from coreos/uint

*: change all ID to uint64
Merge pull request #972 from tazjin/etcd-security-docs

Documentation: Clarify etcd security documentation
discovery: add ability to proxy discovery requests