Extract form validation and queryset filtering from caching

[rpkilby]

I was thinking about #740 a while back, and realized that overriding the FilterSet.qs property is slightly awkward, due to the _qs caching. In the case of #740, Prefetch calls can only occur once, as duplicate calls can raise an exception. If you want to cache the prefetch, a correct solution looks like the following:

@property
def qs(self):
    # ensure that we haven't already cached, as this also implies prefetching
    already_prefetched = hasattr(self, '_qs')
    super().qs
    if self.is_bound and not already_prefetched:
        self._qs = self._qs.prefetch_related(...)
    return self._qs

Overriding filter_queyset() is a little more intuitive, as it would only be executed once.

def filter_queryset(self, queryset):
    qs = super().filter_queryset(queryset)
    return qs.prefetch_related(...)

[codecov-io]

Codecov Report

Merging #783 into develop will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #783      +/-   ##
===========================================
- Coverage    98.36%   98.35%   -0.01%     
===========================================
  Files           15       15              
  Lines         1220     1219       -1     
===========================================
- Hits          1200     1199       -1     
  Misses          20       20

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <100%> (ø)	⬆️

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[rpkilby]

Temporarily closing in favor of alternative approach.