Memory leak in DB2 Addon

[mengxumx]

istoredp.js calls DB2 addon to run XMLSERVICE requests. But there is a memory leak issue in the new async DB2 driver for Node.js v4/v6. To resolve the issue, we need to replace the code "delete stmt/conn" with "stmt.close()/conn.close()" to explicitly clean up.

When the DB2 addon fix is released, the istoredp.js file should be updated accordingly.

[Musse]

migrated issue to: https://github.com/IBM/nodejs-itoolkit/issues/5