Enable source reader throttling on hot conductor backlogs

[ZohebV]

I have code that does source reader throttling when a hot conductor is backlogged. However it is not enabled, as it hasn't been tested.

A test will require 2 machines/JVMs each reading from a source. On of the JVMs should backlog an a key and the test must make sure that the source reader on the other JVM slows down.

[sunweik]

We can work together on this. I can setup test system on my local box, but I need your help on verification of the code.