Fix use-after-free in keepalive_timer and rework stop_client()

The bug reproduced if fio was temporarily stopped with SIGSTOP
during write test and then resumed after 10 seconds. In this case
"pings" were failed for all clients and fio process crashed with
'use-after-free' in keepalive_timer. It happened because it called
stop_client while having a live iterator to the map.

src/msgr_send.cpp

@@ -180,7 +180,7 @@ void osd_messenger_t::handle_send(int result, osd_client_t *cl)
     cl->refs--;
     if (cl->peer_state == PEER_STOPPED)
     {
-        if (!cl->refs)
+        if (cl->refs <= 0)
         {
             delete cl;
         }