Set request timeout to EXCHANGE_LIFETIME of CoAP.

Set request timeout to EXCHANGE_LIFETIME of CoAP.

EXCHANGE_LIFETIME value is 247 seconds.

Change-Id: I95b896e8459200fbd1a02598ea1537c96a8eccef
Signed-off-by: Soemin Tjong <stjong@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18591
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Way Vadhanasin <wayvad@microsoft.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
(cherry picked from commit b024d2ee727fddedc2d6290dfcb5c49433e30523)
Reviewed-on: https://gerrit.iotivity.org/gerrit/18835
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>