Investigate/replace modulestore BranchSettingMixin thread_cache

[ormsbee]

This is not async-safe:

openedx-platform/xmodule/modulestore/draft_and_published.py

Lines 20 to 41 in 628c980

	class BranchSettingMixin:
	"""
	A mixin to manage a module store's branch setting.
	The order of override is (from higher precedence to lower):
	1. thread-specific setting temporarily set using the branch_setting contextmanager
	2. the return value of the branch_setting_func passed into this mixin's init method
	3. the default branch setting being ModuleStoreEnum.Branch.published_only
	"""
	def __init__(self, *args, **kwargs):
	"""
	:param branch_setting_func: a function that returns the default branch setting for this object.
	If not specified, ModuleStoreEnum.Branch.published_only is used as the default setting.
	"""
	self.default_branch_setting_func = kwargs.pop(
	'branch_setting_func',
	lambda: ModuleStoreEnum.Branch.published_only
	)
	super().__init__(*args, **kwargs)
	# cache the branch setting on a local thread to support a multi-threaded environment
	self.thread_cache = threading.local()