Made os::system async signal safe.

Review: https://reviews.apache.org/r/20127
Added a few methods for net::MAC and net::IP.

Review: https://reviews.apache.org/r/19982
Updates download version listed in Getting Started guide to 0.18.0
Fixed process::await and process::collect to take const references.

Review: https://reviews.apache.org/r/20231
Added a metrics API.

Review: https://reviews.apache.org/r/18718