ACCUMULO-2849 Add scanner batch size and write delay configurability to mem stress test

This commit adds one new option each to the Scan and Write components of the memory
stress test. The --scan-batch-size option for Scan configures the batch size of the
scanner it uses, while the --write-delay option for Write allows for a delay between
each mutation added to its batch writer. The reader.sh and writer.sh scripts are
updated for easy configuration of these new options.
ACCUMULO-2827: HeapIterator optimization. Assumes it's more probable that the next entry in a merge comes from the current iterator. Also switching from PriorityBuffer to PriorityQueue<> to avoid unsafe casts

Signed-off-by: Keith Turner <kturner@apache.org>
Merge branch '1.5.2-SNAPSHOT' of https://git-wip-us.apache.org/repos/asf/accumulo into 1.5.2-SNAPSHOT