Implement RCU in Gatekeeper

The function `gk/fib.c:synchronize_gk_instances()` is analogous to RCU's `synchronize_rcu()`, which suggests that we may be able to profit from RCU.

We should consider whether Gatekeeper could profit from RCU for this function and others, and whether it's worth porting RCU to Gatekeeper. If so, implement it.

This feature should remove the relevant `XXX` in `gk/fib.c`.