OPENDJ-2343: minor usability improvements to backendstat tool

Renamed sub-commands to use terminology which is more familiar to end-
users:

High-level sub-commands:

* list-backends - lists pluggable backends
* list-base-dns - lists base DNs of a pluggable backend
* list-indexes - lists all indexes within a backend/baseDN
* show-index-status - shows summary of attribute indexes within
                      a backend/baseDN
* dump-index - dumps an index and decodes key/values

Low-level sub-commands:

* list-raw-dbs - lists underlying databases within a backend
* dump-raw-db - dumps raw hex content of a database.

In addition, command line options have been updated to avoid terms like
"storage", "tree", "entry container", and "root container".