gobject: Drop atomics for interface lookup
Looking up interfaces used to require a lock, since they can (in theory, at least) be added at runtime.
In 2009, Alex replaced the locks by an atomic rcu-style array, see gatomicarray.[hc]
In 2012, Allison prohibited adding interfaces after class_init, with a warning (commit c2055f22).
Therefore, there should not be any need for atomics when looking up interfaces after class_init - the array will not change underneath us.
Edited by Marco Trevisan