Modify discoveryMap's cancel logic in order to keep data intergrity.

Modify discoveryMap's cancel logic in order to keep data intergrity.

- lock guard mutex before erasing data in discoveryMap.

Change-Id: I71f2a63797e417148a670a1d9e880d0c2a6515d4
Signed-off-by: doil.kwon <doil.kwon@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3029
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>
(cherry picked from commit 404a997993bfba1289279683f7b6c6534f3a651d)
Reviewed-on: https://gerrit.iotivity.org/gerrit/3093