Updated Getting Started page for 0.19.0.
Fixed some Javadoc errors.
Improved gate.hpp documentation.

Review: https://reviews.apache.org/r/18988
Created an example LoadGeneratorScheduler to test Master's framework rate limiting feature.

Review: https://reviews.apache.org/r/23250
PortMapping: allow containers to recover even when they were not
managed by Network Isolator previously.

This eases deployment where a slave could be upgraded to use Network
Isolator without removing all the existing tasks.

- Added a new test.
- Moved all portmapping tests to a new file.

Review: https://reviews.apache.org/r/23214