SOC-3626 Failed to migrate from Platform 4.0.1 due to null identity's property

Problem analysis
* Some property of old users can be null. NPE throws when loading the profile of such user.

Fix description
* Try/catch to handle this case.
SOC-3627 [Activity Stream] Slowness in updating Activity Stream after migrating from Platform 4.0.1

Fix description:
* Update activity streams in asynchronous mode.
* Enable the synchronization in ThreadPoolConfig with limited number of pool to improve the speed of loading activities.