Cat API: Improve help and execute help before sending server requests

* Force the cat API classes to have simple help available by extending from a AbstractCatAction
* Use a set binder on guice creation to create the help on node start up
* Make sure that the help/field info is returned without querying any data
expose key and value iterators in ImmutableOpenMap
MetaData to use new iterator in ImmutableOpenMap
Introduce abstract superclass, group cat module bindings, and short-circuit helps.