add doc strings; fix pep8 warning
address some minor issues brought up by the code review process

Use client's session to set POC for test, now that we have the capability to do that.

Use a middleware to set the current POC one time per request, and avoid having to look up the stack for the current request in 'get_current_poc'.

Fetch all overrides for a block at one time.

Speed up the lineage computation by doing some caching.

Unused import.

Get field overrides once per user per block.

Streamline configuration.

Fix poc config tuples

individual students, and a reimplementation of the individual due date
feature.

This work introduces an architecture, used with the 'authored_data'
portion of LmsFieldData, which allows arbitrary field overrides to be
made for fields that are part of the course content or settings (Mongo
data).  The basic architecture is extensible by means of writing and
configuring arbitrary field override providers.

One concrete implementation of a field override provider is provided
which allows for overrides to be for individual students.  This provider
is then used as a basis for reimplementing the individual due date
extensions feature as a proof of concept for the design.

One can imagine writing override providers that provide overrides based
on a student's membership in a cohort or other similar idea.  This work
is being done, in fact, to pave the way for the Personal Online Courses
feature being developed by MIT, which will use an override provider very
much long those lines.