Do not attempt to enumerate extensions before InitMilestone.EXTENSIONS_AUGMENTED

- Spotted a few transient deadlocks (which disappear on restart)
- Root cause is that prior to InitMilestone.EXTENSIONS_AUGMENTED there is no way for Jenkins to safely enumerate the extensions as a plugin could fail and thus require eviction from the list of extensions.
- Correct solution is to enumerate once Jenkins has committed its list of extensions, i.e. InitMilestone.EXTENSIONS_AUGMENTED