Implement the bare minimum of EtcdConsistentStore::NextAvailableSequenceNumber()
Merge pull request #346 from AlCutter/next_available_sequence_number

Implement the bare minimum of EtcdConsistentStore::NextAvailableSequenceNumber()
Put more "using" statements in etcd.cc.

Also use the std::chrono::seconds typedef in the header.
In the etcd watch, wait a bit before starting a new request when there's an error.

This fixes #329.