Put more "using" statements in etcd.cc.

Also use the std::chrono::seconds typedef in the header.
Add a WaitForNotificationWithTimeout method.
Limit the time task_test can run.

A bit cheap, but should be effective (and informative)...
Add support for child tasks to util::Task.
Delete FakeConsistentStore since we can replicate it with EtcdConsistentStore plus FakeEtcdClient.
Merge pull request #347 from AlCutter/bye_bye_fake_consistent_store

Remove FakeConsistentStore - we don't really need it any more now that we can replicate its functionality using EtcdConsistentStore with a FakeEtcdClient.