Added SQL_SERVER_SNAPSHOT value to TransactionIsolationLevel enum

Sql Server supports a non-standard transaction isolation level called
"SNAPSHOT" whose JDBC value is 0x1000 (4096). The new enum value adds
support for this isolation level.

The name "SQL_SERVER_SNAPSHOT" was chosen to reflect the fact that it is
unique to SQL Server to avoid confusion.

Author: maltalex

src/main/java/org/apache/ibatis/session/TransactionIsolationLevel.java

@@ -25,7 +25,12 @@ public enum TransactionIsolationLevel {
   READ_COMMITTED(Connection.TRANSACTION_READ_COMMITTED),
   READ_UNCOMMITTED(Connection.TRANSACTION_READ_UNCOMMITTED),
   REPEATABLE_READ(Connection.TRANSACTION_REPEATABLE_READ),
-  SERIALIZABLE(Connection.TRANSACTION_SERIALIZABLE);
+  SERIALIZABLE(Connection.TRANSACTION_SERIALIZABLE),
+  /**
+  * A non-standard isolation level for Microsoft SQL Server. Defined in
+  * the SQL Server JDBC driver {@link com.microsoft.sqlserver.jdbc.ISQLServerConnection}
+  */
+  SQL_SERVER_SNAPSHOT(0x1000);
 
   private final int level;