HSEARCH-1728 introduce new `metadata_writes_async` configuration property for Infinispan

 - https://hibernate.atlassian.net/browse/HSEARCH-1728

"might improve"? I think it'd be helpful to the user to have some more details, so to give an indication when it might make sense to set this setting. Which are the influencing factors? Also a description of what this setting actually does would be good.