[IOT-1391] fix BLE scan logic which create thread.

[IOT-1391] fix BLE scan logic which create thread.

scan thread was made by unicast/multicast request in BLE client.
it causes a big problem which memory overflow issue related Thread.
so, scan thread will be working as one thread.

Change-Id: I87564f88b67d88781366523ae014c29fe1b13e2e
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12781
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>