Add tooling for working with the read-replica database.

[cpennington]

This migrates the use_read_replica_if_available and read_replica_or_default functions from edx_platform/common/djangoapps/utils/query.py, and augments them with a pair of context managers for asserting that particular chunks of code are read-only (and thus read-replica safe).

Description:

Describe in a couple of sentence what this PR adds

JIRA:

XXX-XXXX

Dependencies:

List dependencies on other outstanding PRs, issues, etc.

Merge deadline:

List merge deadline (if any)

Installation instructions:

List any non-trivial installation instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Reviewers:

• [ ] @edx/arch-review (Required)
• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed
• [ ] PR author is listed in AUTHORS

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns:

List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.

[cpennington]

The build is passing, but not reporting to github (https://travis-ci.org/github/edx/edx-django-utils/builds/666356074). I'm going to merge this.