Added a log line to indicate framework rate limiting is enabled.

Review: https://reviews.apache.org/r/22745
Removed the wait in StartMaster() for the master to get elected before returning because it's no longer necessary.

- Cluster::Masters::start now wait for the master to be recovered, which happens only after it's elected.

Review: https://reviews.apache.org/r/22856
Refactored the querying and parsing of metrics snapshot into a MACRO.

Review: https://reviews.apache.org/r/22639
Added more tests for framework rate limiting.

Review: https://reviews.apache.org/r/22740
Added flags::parse() overload for RateLimits protobuf.

Review: https://reviews.apache.org/r/22758
Added support for optionally throttling the frameworks not specified in RateLimits config.

Review: https://reviews.apache.org/r/22777