Init more thread friendly

- Retrieval of URLForUbiquityContainerIdentifier set to run async
inside init-method.
- Avoided multiple calls to URLForUbiquityContainerIdentifier by using
instance variable
- refined instructions to setup singleton to avoid initializing twice
Init more thread friendly

- Retrieval of URLForUbiquityContainerIdentifier set to run async
inside init-method.
- Avoided multiple calls to URLForUbiquityContainerIdentifier by using
instance variable
- refined instructions to setup singleton to avoid initializing twice