[base] [storage] [engine] Fix various bugs related to the new DataObjectID representation.
[engine] [messages] [scheduling] Add a TaskKillMessage.
[engine] Support for communicating a task heartbeat interval through the executor.
[misc] [unix] Switch to CamIO-style URIs.
[engine] Change some CHECKs to EXPECTs.
[webui] Task kill link.