ACCUMULO-2789 Add stress test with variable cell sizes

Added a test that users can run to stress their instances. They can supply
variable cell sizes (currently row, column family and column qualifier are
supported), and the write clients will generate random data within those size
constraints. Users can also scan random tablets to add futher load onto the
system. A variable number of write and reader clients can be executed to run
the test under a mixed workload.

This test is an acid test-- it is used to verify that an instance can handle
the workload provided by it. It doesn't assert an actual state of the system,
beyond reporting what errors the write and scan clients will see.

See the included package-info.java and/or README.md files for more information.