[SPARK-7698] Cache and reuse buffers in ExecutorMemoryAllocator when using heap allocation

When on-heap memory allocation is used, ExecutorMemoryManager should maintain a cache / pool of buffers for re-use by tasks. This will significantly improve the performance of the new Tungsten's sort-shuffle for jobs with many short-lived tasks by eliminating a major source of GC.

This pull request is a minimum-viable-implementation of this idea.  In its current form, this patch significantly improves performance on a stress test which launches huge numbers of short-lived shuffle map tasks back-to-back in the same JVM.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6227 from JoshRosen/SPARK-7698 and squashes the following commits:

fd6cb55 [Josh Rosen] SoftReference -> WeakReference
b154e86 [Josh Rosen] WIP sketch of pooling in ExecutorMemoryManager