Report to New Relic certain per-request details about the
DjangoXBlockUserStateClient.  The following metrics are reported for the
get_many() call:

xb_user_state.get_many.calls
xb_user_state.get_many.duration
xb_user_state.get_many.blocks_requested
xb_user_state.get_many.blocks_out
xb_user_state.get_many.size
xb_user_state.get_many.<block_type>.blocks_requested
xb_user_state.get_many.<block_type>.blocks_out
xb_user_state.get_many.<block_type>.size

Similarly, for the set_many() call:

xb_user_state.set_many.calls
xb_user_state.set_many.duration
xb_user_state.set_many.blocks_created
xb_user_state.set_many.blocks_updated
xb_user_state.set_many.size
xb_user_state.set_many.<block_type>.blocks_created
xb_user_state.set_many.<block_type>.blocks_updated
xb_user_state.set_many.<block_type>.size

Where <block_type> is one of "chapter", "course", "problem", "video",
etc.

PERF-354

TNL-4449

This abstract class contains most of the fields (aside from the id and
foreign key to StudentModule that the subclasses need to manage).  It
also provides a get_history method that abstracts searching across
multiple backends.

Move router code to openedx/core
We need to use it from cms and lms.
Ensure aws_migrate can be used for migrating both the lms and cms.

Handle queries directed to student_module_history vs default and the
extra queries generated by Django 1.8 (SAVEPOINTS, etc).

Additionally, flag testing classes as multi_db so that Django will
flush the non-default database between unit tests.

Further decouple the foreignkey relation between csm and csmhe

When calling StudentModule().delete() Django will try to delete CSMHE
objects, but naively does so in the database, not by consulting the
database router.

Instead, we disable django cascading deletes and listen for post_delete
signals and clean up CSMHE by hand.

Add feature flags for CSMHE
One to turn it on/off so we can control the deploy.
The other will control whether or not we read from two database tables
or one when searching.

Update tests to explicitly use this get_history method rather than
looking directly into StudentModuleHistory or
StudentModuleHistoryExtended.

Inform lettuce to avoid the coursewarehistoryextended app

Otherwise it fails when it can't find features/ in that app.

Add Pg support, this is not tested automatically.

Injecting call stack manager in CSM/CSMH,introducting @trackit,@wrapt, and refining implementation[PLAT-758]

This required the following changes to the DjangoXBlockUserStateClient
semantics:

1) Changes get/get_many to return XBlockUserState tuples, rather
than state dictionaries or (block_key, state) tuples.
2) Raises DoesNotExist if get_history is called on an XBlock that has
had no data saved to it.
3) Returns XBlockUserState tuples as the results of get_history.

Enable CallStackManager on StudentModule and StudentModuleHistory to capture any use outside of DjangoXBlockUserStateClient