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

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

it is a per-index property. The "infinispan namespace" is for the configuration of the global settings which are applied to the Infinispan CacheManager (the CacheManagerService).
But this attribute can be different on each index, whereas "default" is the *index* default configuration.