Draft: Speed improvements

[srfwx]

Fixes: #658 #625

Various refactoring allows for a substantial increase in performance of the Record initialization.

• _parse_values was heavily refactored to avoid multiple similar checks and unnecessary processing
• in turn, get_return now only needs to work on Record object and drops legacy code from Netbox 2.7
• _endpoint_from_url can work with string instead of using the urlsplit library
• finally caching of apps/endpoints avoid constant reinstantiation of similar objects