Added webui_url to FrameworkInfo and web UI.

Allows frameworks to register a URL to its web UI.

Review: https://reviews.apache.org/r/26125
Replace some asserts with CHECKs in dispatch.

Review:
Added TCP RTT statistics for port mapping network isolator.

Review: https://reviews.apache.org/r/26090
Converted diagnosis socket Info to a struct.

Review: https://reviews.apache.org/r/26290