[20160712212601] Critical Log Reliability Improvements

This update adds critical resiliency improvements to the Convox logging pipeline.

Recently we discovered that convox/agent 0.69 could panic due to a concurrency bug. When this happened, the agent would be restarted and another bug prevented log collection from running containers.

This update fixes both issues. See https://github.com/convox/agent/pull/26 for more details.