Call rcu_barrier earlier in the destructor

If a call_rcu thread is running, there is a possible race condition
where the destructors run before all call_rcu callbacks have finished
running. This can happen, for example, if the call_rcu callback tries to
log something after the logging context has been torn down.

In !10394, we tried to counter this by explicitely creating a call_rcu
thread an shutting it down before running the destructors, but it is
possible for things to "slip" and end up on the default call_rcu thread.

As a quickfix, this commit moves an rcu_barrier() that was in the mem
context destructor earlier, so that it "protects" all libisc
destructors.

lib/isc/mem.c

@@ -434,10 +434,9 @@ isc__mem_initialize(void) {
 
 void
 isc__mem_shutdown(void) {
+	/* should be called after an rcu_barrier() */
 	bool empty;
 
-	rcu_barrier();
-
 	isc__mem_checkdestroyed();
 
 	LOCK(&contextslock);