Improve .NET Core MySQL performance. (#3741)

frameworks/CSharp/aspnetcore/Benchmarks/appsettings.mysql.json

@@ -1,4 +1,4 @@
 {
-  "ConnectionString": "Server=tfb-database;Database=hello_world;User Id=benchmarkdbuser;Password=benchmarkdbpass;Maximum Pool Size=1024;SslMode=None;ConnectionReset=false",
+  "ConnectionString": "Server=tfb-database;Database=hello_world;User Id=benchmarkdbuser;Password=benchmarkdbpass;Maximum Pool Size=1024;SslMode=None;ConnectionReset=false;ConnectionIdlePingTime=900;ConnectionIdleTimeout=0",
   "Database": "mysql"
 }