server: Fix PMIx_Server_Finalize hang

The hang was quite rare and appears as the result of the race condition
between PMIx progress thread and main thread calling
PMIx_Server_finalize.

The following sequence is possible:

| main thread   | Progress thread   |
|               | while(ev_active){ |
| ev_active=0   |                   |
| ev_break_loop |                   |
|               | ev_loop()         |

According to libevent manual, in this situation, libevent will
ignore ev_break_loop as it wasn't in the loop at the time
ev_break_loop() was called (see (b) in the libevent excerpt below)
So the progress thread will enter the loop and hang.

To fix this use event_base_loopexit that have desired behavior
(See section (a) of the excerpt below)

**excerpt from the libevent manual**:
```
...
Note also that event_base_loopexit(base,NULL) and event_base_loopbreak(base)
act differently when no event loop is running:

(a) loopexit schedules the next instance of the event loop to stop right
after the next round of callbacks are run (as if it had been invoked with
EVLOOP_ONCE)

(b) whereas loopbreak only stops a currently running loop, and has no
effect if the event loop isn’t running.
...
```

Signed-off-by: Artem Polyakov <artpol84@gmail.com>

src/runtime/pmix_progress_threads.c

@@ -3,6 +3,8 @@
  * Copyright (c) 2015      Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2017-2019 Research Organization for Information Science
  *                         and Technology (RIST).  All rights reserved.
+ * Copyright (c) 2019      Mellanox Technologies, Inc.
+ *                         All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -239,7 +241,7 @@ static void stop_progress_engine(pmix_progress_tracker_t *trk)
     trk->ev_active = false;
     /* break the event loop - this will cause the loop to exit upon
        completion of any current event */
-    pmix_event_base_loopbreak(trk->ev_base);
+    pmix_event_base_loopexit(trk->ev_base);
 
     pmix_thread_join(&trk->engine, NULL);
 }