enhance: increate tantivy index worker and memory budget for json key stats for buliding index (#40912)

issue: #40897

This PR, combined with https://github.com/milvus-io/milvus/pull/40898,
makes tantivy total duration decrease roughly
from 6.3s to 1.68s for the case in the issue.

Signed-off-by: SpadeA <tangchenjie1210@gmail.com>

internal/core/src/index/JsonKeyStatsInvertedIndex.cpp

@@ -280,8 +280,9 @@ JsonKeyStatsInvertedIndex::JsonKeyStatsInvertedIndex(
             path_.c_str(),
             false,
             false,
-            1,
-            json_stats_tantivy_memory_budget);
+            // To speed up index building process.
+            4,
+            4 * 64 * 1024 * 1024);
     }
 }