node shutdown: make close() syncronized

An example scenario where this will help:

When the node is shutdown via api call
(https://github.com/elasticsearch/elasticsearch/blob/master/src/test/java/org/elasticsearch/test/ExternalNode.java#L219 )
then the call returns immediately even if the node is not actually shutdown yet
(https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/action/admin/cluster/node/shutdown/TransportNodesShutdownAction.java#L226).
If at the same time the proces is killed, then the hook that would usually prevent
uncontrolled shutdown
(https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/bootstrap/Bootstrap.java#L75)
has no effect: It again calls close() which might then just return
for example because one of the lifecycles was moved to closed already.

The bwc test FunctionScoreBackwardCompatibilityTests.testSimpleFunctionScoreParsingWorks
failed because of this. The translog was not properly
written because if the shutdown was called via api, the following process.destroy()
(https://github.com/elasticsearch/elasticsearch/blob/master/src/test/java/org/elasticsearch/test/ExternalNode.java#L225)
killed the node before the translog was written to disk.

closes #7885

src/main/java/org/elasticsearch/node/internal/InternalNode.java

@@ -312,7 +312,10 @@ public Node stop() {
         return this;
     }
 
-    public void close() {
+    // During concurrent close() calls we want to make sure that all of them return after the node has completed it's shutdown cycle.
+    // If not, the hook that is added in Bootstrap#setup() will be useless: close() might not be executed, in case another (for example api) call
+    // to close() has already set some lifecycles to stopped. In this case the process will be terminated even if the first call to close() has not finished yet.
+    public synchronized void close() {
         if (lifecycle.started()) {
             stop();
         }