Bug 875400 - Add-on support and hashing changes for Environment.
Improvements since first review.

Log failure in onUpgrade.
Load addons from JSON.
Try not to NPE when putting envs.
Generator debugging.
Don't blow away profile creation time.
Ensure that DatabaseEnvironment inits its own id.
Log if our add-on set is empty.
Debug logging in EnvBuilder
Test and fix shallowCopyObject.
Advance environment cursor.
Refuse to record events with invalid environment.
Tweaks for environment registration.
Safer handling of invalid environments.
Don't log profile path unless logging PII.
Ignore some add-ons.
Review comments.
Bug 875400 - Add-on support and hashing changes for Environment. r=nalexander