examples/daemon: re-organize sample handler

split commands to show one weave feature at a time:
- hello: command handling with args and results
- ping: device state
- countdown: long running command w/ progress

Bug: 24992153
Change-Id: Ib901239170287225c239c224c4fc9aa0b4112856
Reviewed-on: https://weave-review.googlesource.com/1347
Reviewed-by: Johan Euphrosine <proppy@google.com>