Add ContainerInfo and Volumes protobufs.

Review: https://reviews.apache.org/r/24349
Made value in CommandInfo optional and added argv.

Review: https://reviews.apache.org/r/24619
Refactored the protobuf message comparison logic.

Review: https://reviews.apache.org/r/24631
Updated health check to use the new CommandInfo.

Review: https://reviews.apache.org/r/24632
Updated mesos containerizer launcher to use the new CommandInfo.

Review: https://reviews.apache.org/r/24633
Updated slave http to use the new CommmandInfo.

Review: https://reviews.apache.org/r/24634
Updated command executor to use the new CommandInfo.

Review: https://reviews.apache.org/r/24635
Fixed bug in io::poll.

When a future returned from process::io::poll is discarded we need to
fully remove the ev_io watchers from libev otherwise the reuse of
those file descriptors can cause untended side effects because they're
still being used by the kernel/libev.

Review: https://reviews.apache.org/r/24194