When releasing the versioned assets work, we stumbled on a problem with old pickled
versions of the StaticContent objects residing in cache, which triggered a bug in the
code. Not wanting to blow away all cached items, we ended up having to revert and add
in some backwards-compatible helper code to ease the transition.

With this, we'll now utilize the version argument that Django's caching interface
allows, in conjunction with a constant value that can be modified when breaking changes
are being made, to let us gracefully ignore older cached course assets.

This is slightly more complicated than it should be since we're using
custom authentication middleware (i.e., not Django's standard
middleware class). We have to check that the session auth hash we have
stored is equal to the request's session auth hash (since the stored
hash is a function of the password). Normally this gets handled in
`django.contrib.auth.get_user`, but due to our caching we don't go
through that function, even in the cache miss case.

ECOM-4288

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Make default_store thread-safe.

fixing unit tests

fixing merge error

fixing xqueue submission issue with unicode url (trial 0.1)

fixing fotmats as commented upon

removing yaml file language selection

Unicode changes to support QRF

removed unnecessary pass in modulestore/init.py

fixing merge error

fixing fotmats as commented upon

removing yaml file language selection

fixing pep8 violations

- fixing pylint violations

pylint violation

fixing line spaces and formats

ignore pylint E1101

remove empty line

fixing pylint violations

 pep8 violations

bulk mail unicode/decode

fix migration error

fix pep8 just to push again

more unicode/decode
Final changes to comments and error messages.

remove discussion caching for now. It might be overkill and I need to figure out how to serialize out the resultset into the cache as writing modules into the cache causes a pickle error

refactor StaticContent id scheme to be the same as moduledata. Also expose a new 'get all content for course' which will be needed to support a asset management page.

Support asset library references in CapaModule. Also some code refactoring/cleanup. Preparing for review and submit