docs: clarify multiple x-conflicts per unit
Merge pull request #834 from robszumski/master

docs: clarify multiple x-conflicts per unit
Merge pull request #839 from jonboulle/reg

agent: remove unnecessary manager from state publisher
agent: update UnitStatePublisher locking + timing

- rename addToCache --> updateCache
- updateCache now publishes _synchronously_ if a change has occurred
- updateCache checks if state existed previously in cache and if not,
  publishes immediately (i.e. considers it a change)
- publishAll runs at most every (TTL/2) since the last PublishAll
- publishAll only locks the cache to copy it; no longer holds lock while
  writing to etcd
- add test coverage for updateCache
- fix some bugs in FakeRegistry