ACCUMULO-2216 Add ShortEach.xml and LongEach.xml randomwalk modules

The added modules are linear, running each nested module once and then ending.
The difference between them is in the maximum time for each nested module to run.
Merge branch '1.4.5-SNAPSHOT' into 1.5.1-SNAPSHOT
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT
Merge branch '1.6.0-SNAPSHOT'