Janus shutdown hook cannot be disabled and logs unwatned warnings

When `JanusGraph` `close()` method is called from another shutdown hook e.g. coming from spring boot application, bogus warnings are logged:

````
WARN [ Thread-11] o.j.g.d.StandardJanusGraph : Failed to remove shutdown hook
java.lang.IllegalStateException: Shutdown in progress
at java.lang.ApplicationShutdownHooks.remove(ApplicationShutdownHooks.java:82) ~[?:1.8.0_144]
at java.lang.Runtime.removeShutdownHook(Runtime.java:239) ~[?:1.8.0_144]
at org.janusgraph.graphdb.database.StandardJanusGraph.removeHook(StandardJanusGraph.java:259) [janusgraph-core-0.2.0.jar:?]
at org.janusgraph.graphdb.database.StandardJanusGraph.close(StandardJanusGraph.java:193) [janusgraph-core-0.2.0.jar:?]
at ...
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_144]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_144]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_144]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_144]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(InitDestroyAnnotationBeanPostProcessor.java:366) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeDestroyMethods(InitDestroyAnnotationBeanPostProcessor.java:325) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessBeforeDestruction(InitDestroyAnnotationBeanPostProcessor.java:154) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DisposableBeanAdapter.destroy(DisposableBeanAdapter.java:253) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroyBean(DefaultSingletonBeanRegistry.java:578) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroySingleton(DefaultSingletonBeanRegistry.java:554) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.destroySingleton(DefaultListableBeanFactory.java:961) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroyBean(DefaultSingletonBeanRegistry.java:571) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroySingleton(DefaultSingletonBeanRegistry.java:554) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.destroySingleton(DefaultListableBeanFactory.java:961) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroySingletons(DefaultSingletonBeanRegistry.java:523) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.destroySingletons(FactoryBeanRegistrySupport.java:230) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.destroySingletons(DefaultListableBeanFactory.java:968) [spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.destroyBeans(AbstractApplicationContext.java:1032) [spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.doClose(AbstractApplicationContext.java:1008) [spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.context.support.AbstractApplicationContext$2.run(AbstractApplicationContext.java:929) [spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
````

Either there should be an option to disable Janus shutdown hook completely (via configuration property or Java api) or the warning should not be logged when application shutdown is in progress.

Start with StandardJanusGraph.close() and removeHook() at the locations shown in the stack trace, then examine how shutdown-hook removal behaves during JVM shutdown. Reproduce the Spring Boot shutdown sequence and determine whether completion should suppress the warning or expose a way to disable the hook. Done means the shutdown no longer emits the bogus warning or the hook can be disabled as specified.

Written by the indexing model from the issue text.