Prevent a shutdown hook from being removed when a shutdown is in progress (#689)

Signed-off-by: Tomas Hofman <[email protected]>

Author: TomasHofman

jaeger-core/src/main/java/io/jaegertracing/internal/JaegerTracer.java

@@ -133,6 +133,7 @@ protected JaegerTracer(JaegerTracer.Builder builder) {
       shutdownHook = new Thread() {
         @Override
         public void run() {
+          shutdownHook = null;
           JaegerTracer.this.close();
         }
       };