refactor _generate_items_for_subtask not to chunk queries

use django's iterator methor for querysets

remove bulk email query settings

use read_replica if available, since this is read-only

update changelog

This commit adds the non-courseware lms/djangoapps and lms/lib.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

When creating an instructor task total_num_items may change between the
time it and the number of subtasks is calculated and the time the
subtasks are actually queued (all of this cannot happen in one transaction).
In such a case the extra items are distributed among the subtasks of the
last query.

LMS-2090

Add cache-based locking to subtasks to ensure that the same task is not running in two workers at the same time.

Add cache-based locking to subtasks to ensure that the same task is not running in two workers at the same time.