Added a JSON::Object::find.

Review: https://reviews.apache.org/r/24541
Changed Try::error and Result::error to return const references.

Review: https://reviews.apache.org/r/18489
Added missing 'defer' helpers.

Fixes compilation issue when building with gcc 4.4, see MESOS-1692 for
more details.

Review: https://reviews.apache.org/r/24569