Added "per-framework-principal" counters for messages from a scheduler to the master.

Review: https://reviews.apache.org/r/22316
Changed RateLimiter to work directly with 'double permitsPerSecond'.

Review: https://reviews.apache.org/r/22424
Created framework rate limits protobuf object which is loaded as JSON through master flags.

Review: https://reviews.apache.org/r/22425
Changed MessageEvent and ExitedEvent to be copyable.

Review: https://reviews.apache.org/r/22426
Implemented framework API rate limiting.

Review: https://reviews.apache.org/r/22427