Also ignore the docgen-generated directories under doc/.

Delay beginning most writes until we enter poll()

-Rename namespace to grpc_health->grpc to match spec
-Proper use of NOT_FOUND status code
-Improve testing
-Add source distribution to artifact build

Also removes an old-now-spurious `make` invocation from the docgen.py
helper script.

The examples under <repo_root>/examples now rely on the released
versions. Those under src/objective-c/examples, as well as the tests,
rely on protoc and the plugin as compiled from head.

Now reasonable defaults are auto-detected by platform (and by specific
Python implementation).