Use session isolation level by default. Fixes #801

Author: bgrainger

src/MySqlConnector/Core/StandardEnlistedTransaction.cs

@@ -29,7 +29,7 @@ protected override void OnStart()
 				_ => "repeatable read",
 			};
 
-			using var cmd = new MySqlCommand($"set transaction isolation level {isolationLevel};", Connection);
+			using var cmd = new MySqlCommand($"set session transaction isolation level {isolationLevel};", Connection);
 			cmd.ExecuteNonQuery();
 
 			var consistentSnapshotText = Transaction.IsolationLevel == IsolationLevel.Snapshot ? " with consistent snapshot" : "";

src/MySqlConnector/MySql.Data.MySqlClient/MySqlConnection.cs

@@ -71,7 +71,7 @@ private async ValueTask<MySqlTransaction> BeginDbTransactionAsync(IsolationLevel
 				_ => throw new NotSupportedException("IsolationLevel.{0} is not supported.".FormatInvariant(isolationLevel))
 			};
 
-			using (var cmd = new MySqlCommand($"set transaction isolation level {isolationLevelValue};", this))
+			using (var cmd = new MySqlCommand($"set session transaction isolation level {isolationLevelValue};", this))
 			{
 				await cmd.ExecuteNonQueryAsync(ioBehavior, cancellationToken).ConfigureAwait(false);