add assert for active extstore sub-objects

I'm pretty sure it's impossible to hit that condition, but I want an
assert during burn-in. Tried writing unit tests to cause and traced
through the code without luck already.

proto_proxy.c

@@ -611,6 +611,7 @@ void proxy_finalize_rctx_cb(io_pending_t *pending) {
     // TODO: need to remove from stack if subtype is p->active
 
     if (p->io_sub_type == IO_PENDING_TYPE_EXTSTORE) {
+        assert(p->active == false);
         if (p->hdr_it) {
             // TODO: lock once, worst case this hashes/locks twice.
             if (p->miss) {

thread.c

@@ -525,8 +525,6 @@ static void *worker_libevent(void *arg) {
         event_base_loop(me->base, EVLOOP_ONCE);
         // Run IO queues after the event loop to catch things like
         // re-submissions from proxy callbacks.
-        // TODO: check for latency impact, as we might need to do this after
-        // each event processed instead of after each base loop run.
         thread_io_queue_submit(me);
 #ifdef PROXY
         if (me->proxy_ctx) {