Pass through X-Etcd-Index
Use the std::chrono types and functions for libevent.
Create an PeriodicClosure class.
Improve the interface of ReadPrivateKey.
Use int64_t for the sequence number everywhere.
Add a util::Executor interface, for running closures.
Add support for sparse entries in FileDB.
Add support for sparse entries in SQLiteDB.
Add a test for the sparse entries.