rename ValueChange to ValueChanged to match BuildStructureChanged

Events in past tense, is the principle.
Merge pull request #101 from sbt/wip/havocp-drop-boot-events

Remove NowListeningEvent, Stopped, NeedRebootEvent
Merge pull request #100 from sbt/wip/havocp-rename-value-change

rename ValueChange to ValueChanged to match BuildStructureChanged
Attempt to reply exactly once to all requests

This allows requesters to block for an "ack" or ErrorResponse if they see
fit. Without the non-error ack, there's no way to know whether an error
is going to arrive later. So this is good general practice to always
have a "yes I got it" reply even if for now we never send back an
error for a particular request.