Synchronise access to `ServiceLocator`
Synchronise access to `ServiceLocator`

Added a serial queue to `ServiceLocator` to synchronise access to the
internal dictionaries, avoiding data races.

In the future this sync mechanism should be replaced by a lighter weight
one, when more concurrency primitives are natively available in Swift
(hopefully in Swift 5 🙏).

Mindera/Alicerce,Mindera/Alicerce,Mindera/Alicerce