HSEARCH-1723 Allow testing helper SearchFactoryHolder to start multiple cluster nodes

Also allow more time in testhelper Conditions for slower machines
and reviewing default scale settings for BackendStressTest
HSEARCH-1728 Create a configuration attribute to allow usage of the Infinispan Directory option writeFileListAsynchronously
HSEARCH-1728 Test for new Infinispan configuration attribute 'metadata_writes_async'