Verify that pmap has a readahead memory leak

Pmap uses a thread to add tasks to the queue. That thread is not
bounded by anything, so given lots of potential tasks, it can realize
lots of lazy input.
Fix readahead memory leak
Clearer comments & code for fix #17

This addresses Seba's comments.
Simplify calling of pmap-boilerplate and pmap-core

The other changes to pmap-core have made this simplification possible.
Slight readability/doc improvements
Merge pull request #17 from TheClimateCorporation/fix-readahead

Fix readahead memory leak