HBASE-20276 restore original shell REPL functionality where commands can return results
* makes commands always pass any results back to hirb
* print warning if hirb is given the --return-values flag
* add some docs on how to avoid the console clutter that HBASE-15965 sought to address
* add an upgrade section note about this change.
* cleanup where the get_splits command does its printing so there's a building block that doesn't print
* some rubocop suggested tweaks and opt-out for classlength check on table and shell classes.
Signed-off-by: Mike Drob <mdrob@apache.org>