Refactor open/close -> prepare/cleanup -- decouple more from scrapli

Title basically...

cfg connections now need to have prepare called on them (context manager will do this for you), but unless dedicated_connection is set to True they now basically assume you are going to leave the scrapli connection alone (i.e. scrapli-cfg will not open/close the underlying scrapli conn w/out that dedicated_connection arg).

on_open --> on_prepare and the default callable have been removed -- too much magic, users can/should provide an on_prepare to handle whatever they need for their environment.

Closer to a stable public release (2021.07.30 of course)