Avoid count(*) query in management command.

Count(*) is very slow on large tables.

problem_builder/management/commands/copy_deprecated_course_id.py

-import math
 import time
 from django.core.management.base import BaseCommand
 from problem_builder.models import Answer
@@ -29,18 +28,21 @@ class Command(BaseCommand):
         batch_size = options['batch_size']
         sleep_time = options['sleep']
         queryset = Answer.objects.filter(course_key__isnull=True)
-        batch_count = int(math.ceil(queryset.count() / float(batch_size)))
 
         self.stdout.write(
             "Copying Answer.course_id field into Answer.course_key in batches of {}".format(batch_size)
         )
 
-        for batch in xrange(1, batch_count + 1):
-            for answer in queryset[:batch_size]:
+        idx = 0
+        while True:
+            idx += 1
+            batch = queryset[:batch_size]
+            if not batch:
+                break
+            for answer in batch:
                 answer.course_key = answer.course_id
                 answer.save()
-            self.stdout.write("Processed batch {} of {}".format(batch, batch_count))
-            if batch != batch_count:
-                time.sleep(sleep_time)
+            self.stdout.write("Processed batch {}".format(idx))
+            time.sleep(sleep_time)
 
         self.stdout.write("Successfully copied Answer.course_id into Answer.course_key")