net: lwm2m: Thread safe lwm2m registry

Created the mutex registry_lock to:
- protect read and write operations
- protect the registry.
Only partially finished as the functions like lwm2m_engine_get_obj
warrants a larger refactoring to completely thread safe the registry.

Signed-off-by: Ola Tangen Kulseng <ola.kulseng@nordicsemi.no>