Commits on May 14, 2019

1. 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>
   (cherry picked from commit f652ae1)
   
   Conflicts:
   	src/runtime/pmix_progress_threads.c

   

   artpol84 authored and karasevb committed May 14, 2019
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