Make models set() and get() inherently thread safe

[polfeliu]

[create-issue-branch]

Branch issue-1-Make_models_set_and_get_inherently_thread_safe created!

[polfeliu]

Probably a mutex to the store variable is enough

[polfeliu]

Also important (even more so) for rq_getattr.
__getattribute__ needs to also be overriden to acquire the mutex first