Add tools for ContextDefinition create and cleanup (#33)

Modified ContextDefinition to implement KeywordExecutable so that
the user can use `accumulo create-context-definition` to create the
json required for the ContextDefinition file.

Added a JMX MXBean that will return the list of files in the local cache
directory that are referenced by classloaders created by the factory
in this module and are still in use in the JVM. Users can connect to
the JMX MXBean to retrieve the list to aid in the cleanup of the local
cache directory.


Co-authored-by: Christopher Tubbs <ctubbsii@apache.org>
Co-authored-by: Keith Turner <kturner@apache.org>