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

[srfwx]

@arthanson the size of this PR may have gotten a little bit out of hand.
I assume you may want to break it into into smaller changes.

As a summary here is what's been implemented so far:

• Cache of Endpoints on App
• Cache nested Records on Endpoints
• Adds class ValueRecord for dict that don't include an endpoint (contrary to class Record)
• Improves speed of computation of endpoint from URL
• Lazy computation of Record.endpoint
• Improves value parsing performance and readability

Please let me know what you think and what you'd like to possibly see as individual PR.