log: stderr, not stdout

Fix #841
doc link update
Merge pull request #844 from igm/patch-1

doc ref link update
Merge pull request #843 from bcwaldon/841

log: stderr, not stdout
registry: fix state-related bugs in FakeRegistry
agent: refactor UnitStatePublisher locking + timing

- rename addToCache --> updateCache
- split logic in publishAll into pruneCache and Run
- updateCache checks if state existed previously in cache and if not,
  publishes immediately (i.e. considers it a change)
- Run waits TTL/2 between publishing entire state
- Run only locks the cache to copy it; no longer holds lock while
  publishing to etcd
- add test coverage for updateCache & pruneCache
- fix some bugs in FakeRegistry
agent: dump UnitStatePublisher cache on SIGUSR1