ACCUMULO-841 Refactor randomwalk State class

* Create new Environment object, separate from State, for config props and client objects
* Update most randomwalk test nodes and framework to use Environment
* Eliminate State visit counting, which is unused and redundant with Module's maxHops
* Move getMapReduceJars() method to Node
* Move getPid() method out of State
* Eliminate getCredentials() which ends up not useful
* Reduce visibility to several methods in State and Environment