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.
Add an etcd watcher class.

Also a few cleanups I forgot in previous commits, like not using a temporary
object when using UpdateLeader, and using an unique_ptr in RequestDone.