feat(django_getter): use function cache to increase performance

[yorickr]

Hi!

After using ninja a lot, I noticed some performance benefit could be made in DjangoGetter, by caching the actual function being called and using a lookup, instead of doing up to 4 checks each time.

This MR implements this simple cache.
I didn't specifically add tests for this, as schema is tested very well throughout the rest of the tests. Let me know if you'd like to see this differently.

Let me know what you think!

Some data:

Old code: Average time across 20 runs of 100k nested objects is 5.516484143049911
New code: Average time across 20 runs of 100k nested objects is 4.045871758400063
This is 1.5s saved, or 27% saved.

Old code: Average time across 20 runs of 10k nested objects is 0.4554609017498933
New code: Average time across 20 runs of 10k nested objects is 0.35757346044997573
This saved 22%.