Instead of loading all QuerySet results in memory, count the number of
entries. This adds an extra query when list() or tuple() is called on the
choices (because both call __len__() then __iter__()) but uses less
memory since the QuerySet results won't be cached. In most cases, the
choices will only be iterated on, meaning that __len__() won't be called
and only one query will be executed.
When __len__() is called (e.g. when casting to list or tuple), the
QuerySet is evaluated and the result cache populated. iterator()
shouldn't be called on the QuerySet after that, as it would reset the
result cache and trigger a second query.
