optimization to use the existing buffer in blockwise-transfer.

optimization to use the existing buffer in blockwise-transfer.

if we have to update payload data, we always allocate new memory.
so we modified the code to use the existing memory if one exists.
This patch addresses the first part of IOT-690.

Change-Id: I04ca551273caf02f783c62a76f11f7e07409d8dd
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2503
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>