Improve LimitOffsetPagination.get_count to avoid extra Count query

[jurrian]

I've traced back in blame that LimitOffsetPagination.get_count has used queryset.count() essentially since the start. However, this will cause an extra query and it would be more efficient to always use len() when we are going to use the queryset anyway (source).
So this:

    def get_count(self, queryset):
        """
        Determine an object count, supporting either querysets or regular lists.
        """
        try:
            return queryset.count()
        except (AttributeError, TypeError):
            return len(queryset)

Needs to become this:

    def get_count(self, queryset):
        """
        Determine an object count, supporting either querysets or regular lists.
        """
        return len(queryset)

If tested this and it works. If there are no objections I can create an issue and a PR.