So we are always returning a set of unique Endpoints.

And remove the unused StringSet type.

Includes a merge of radiod and redirectord, updates to more modern Go
idioms, adoption of the higher level etcd concurrency primitives, and
a lot of other minor implementation changes.

The refactor is still incomplete: the daemons won't compile and the
code to start the RPC servers is missing.

This change involves a refactoring of the DNS redirector code, which is
now a bit cleaner and has proper tests.

* Track ModifiedIndex, preventing missed updates
* Properly handle errors in presence protocol

Ensure that the Syncer listens on all interesting updates (added
compareAndSwap and compareAndDelete).