Make blob cache have async durability (#66687)

The blob cache for searchable snapshots used default sync durability.
Indexing into this often comes in bursts (multiple files for a shard or
multiple shards for an index or even multiple indexes being mounted) and
since it is immutable and easily reconstructed, this commit changes
durability to async. This improves mount performance when blob cache
index is co-located with the searchable snapshot(s).

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshots.java

@@ -39,6 +39,7 @@ import org.elasticsearch.index.engine.EngineFactory;
 import org.elasticsearch.index.engine.ReadOnlyEngine;
 import org.elasticsearch.index.store.SearchableSnapshotDirectory;
 import org.elasticsearch.index.store.Store;
+import org.elasticsearch.index.translog.Translog;
 import org.elasticsearch.index.translog.TranslogStats;
 import org.elasticsearch.indices.SystemIndexDescriptor;
 import org.elasticsearch.indices.recovery.SearchableSnapshotRecoveryState;
@@ -392,6 +393,7 @@ public class SearchableSnapshots extends Plugin implements IndexStorePlugin, Eng
             .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
             .put(IndexMetadata.SETTING_AUTO_EXPAND_REPLICAS, "0-1")
             .put(IndexMetadata.SETTING_PRIORITY, "900")
+            .put(IndexSettings.INDEX_TRANSLOG_DURABILITY_SETTING.getKey(), Translog.Durability.ASYNC)
             .put(DataTierAllocationDecider.INDEX_ROUTING_PREFER, DATA_TIERS_PREFERENCE)
             .build();
     }