Wrap accesses to EtcdClient::leader_.
Add an HttpConnection::Clone method.
Implement AssignSequenceNumber() method in Etcd consistent store.
Merge pull request #228 from AlCutter/etcd_seq

Implement AssignSequenceNumber() method in Etcd consistent store.
Make HttpRequest extend the lifetime of its HttpConnection.

Also make HttpConnection::Clone return a shared_ptr instead of a raw pointer.
Slimfast DB