[SPARK-8977] [STREAMING] Defines the RateEstimator interface, and impements the RateController

Based on #7471.

- [x] add a test that exercises the publish path from driver to receiver
- [ ] remove Serializable from `RateController` and `RateEstimator`

Author: Iulian Dragos <jaguarul@gmail.com>
Author: François Garillot <francois@garillot.net>

Closes #7600 from dragos/topic/streaming-bp/rate-controller and squashes the following commits:

f168c94 [Iulian Dragos] Latest review round.
5125e60 [Iulian Dragos] Fix style.
a2eb3b9 [Iulian Dragos] Merge remote-tracking branch 'upstream/master' into topic/streaming-bp/rate-controller
475e346 [Iulian Dragos] Latest round of reviews.
e9fb45e [Iulian Dragos] - Add a test for checkpointing - fixed serialization for RateController.executionContext
715437a [Iulian Dragos] Review comments and added a `reset` call in ReceiverTrackerTest.
e57c66b [Iulian Dragos] Added a couple of tests for the full scenario from driver to receivers, with several rate updates.
b425d32 [Iulian Dragos] Removed DeveloperAPI, removed rateEstimator field, removed Noop rate estimator, changed logic for initialising rate estimator.
238cfc6 [Iulian Dragos] Merge remote-tracking branch 'upstream/master' into topic/streaming-bp/rate-controller
34a389d [Iulian Dragos] Various style changes and a first test for the rate controller.
d32ca36 [François Garillot] [SPARK-8977][Streaming] Defines the RateEstimator interface, and implements the ReceiverRateController
8941cf9 [Iulian Dragos] Renames and other nitpicks.
162d9e5 [Iulian Dragos] Use Reflection for accessing truly private `executor` method and use the listener bus to know when receivers have registered (`onStart` is called before receivers have registered, leading to flaky behavior).
210f495 [Iulian Dragos] Revert "Added a few tests that measure the receiver’s rate."
0c51959 [Iulian Dragos] Added a few tests that measure the receiver’s rate.
261a051 [Iulian Dragos] - removed field to hold the current rate limit in rate limiter - made rate limit a Long and default to Long.MaxValue (consequence of the above) - removed custom `waitUntil` and replaced it by `eventually`
cd1397d [Iulian Dragos] Add a test for the propagation of a new rate limit from driver to receivers.
6369b30 [Iulian Dragos] Merge pull request #15 from huitseeker/SPARK-8975
d15de42 [François Garillot] [SPARK-8975][Streaming] Adds Ratelimiter unit tests w.r.t. spark.streaming.receiver.maxRate
4721c7d [François Garillot] [SPARK-8975][Streaming] Add a mechanism to send a new rate from the driver to the block generator