cache when looking for schemas

[rpiazza]

Some of the examples for the incident extension were taking forever to validate.

Looking for schemas, especially $ref ones to github (e.g., timestamp) were being asked for time and time again.

I cache them in a mapping so they only have to be looked up once.

Similar for looking for extension schemas

[rpiazza]

Many of the tests are failing - But I don't know why - lots of "invalid" tests look invalid to me, so why don't they pass?

[rpiazza]

Ignore the previous note - my changes caused the errors, but now they are all fixed.