Fixes BLD-1104
Merge pull request #3962 from edx/dcs/BLD-1104

Fixes BLD-1104
Fix performance regression
Remove course_from_id
don't fetch whole course when the root will do
don't fetch even the root if the id will do
check for definition.data == null
OpaqueKey copy and deepcopy are identity ops
because keys are immutable
pymongo son optimizations
Update to tagged version of pymongo