As a developer I want the multiprocessing Pool class to expose the internal queue size limit in order to better control the task flow in my application.

Consider the following scenarios:
1. The tasks I want to run into the pool require a considerably big amount of data as input (a big XML string for example). A limitless queue and a high number of tasks would cause my application to consume a huge amount of memory, this is unacceptable.
2. I want to always ensure a small amount of tasks queued into the pool so that, if my application crashes, I won't loose much data.

Patch includes code changes, tests and documentation.