Nth attempt at correct shutdown procedure.

If we install an explicit signal handler for SIGTERM and SIGINT and then call
sys.exit ourselves, atexit handlers are run. If we don't call sys.exit from the
handler, ycmd never shuts down. So fixed... I think. We'll see.

Fixes #577... again.

python/ycm/server/ycmd.py

@@ -35,12 +35,11 @@ def YcmCoreSanityCheck():
     raise RuntimeError( 'ycm_core already imported, ycmd has a bug!' )
 
 
-# We need to manually call ServerShutdown for the signals that turn down ycmd
-# because atexit won't handle them.
+# We manually call sys.exit() on SIGTERM and SIGINT so that atexit handlers are
+# properly executed.
 def SetUpSignalHandler():
   def SignalHandler( signum, frame ):
-    import handlers
-    handlers.ServerShutdown()
+    sys.exit()
 
   for sig in [ signal.SIGTERM,
                signal.SIGINT ]: