Better error message for protobuf::write.

Review: https://reviews.apache.org/r/20072
Added 'mesos-usage' for use by external containerizers.

Review: https://reviews.apache.org/r/20070
Added test for slave stopping before containerizer launches.

Review: https://reviews.apache.org/r/20179
Allowed passing headers to http::get/post.

Review: https://reviews.apache.org/r/20275
Updated uses of http::post.

Review: https://reviews.apache.org/r/20278