agent: remove restart command

Other, somewhat random changes:

- Clarify agent/container heartbeat state machine transitions
- Naming convention stuff (request channels in container)
- Code style stuff (all over the place)
agent: cherrypick host resource accounting from master
agent: heartbeat from unknown container yields FORCEDOWN
agent: re-order validation switch statements
container: use new, fewer states in Heartbeat and state machine
container: coordinate shutdown with agent
Merge pull request #23 from soundcloud/agent-restart-redux

agent: remove restart command