Refactor cancel async scan architecture

Refactor cancel async scan architecture

Before: Check cancelled flag on both of client/server side in loop of
        file visitor. It's hard to control response latency consistently.

After : Change client side connection to non-blocking to cancel it
        directly by sending signal to fd. Response latency is consistent
        because client just close connection.

Change-Id: If181eb9984357939b2845b7d03a17dac57a0b9d0
Signed-off-by: Kyungwook Tak <k.tak@samsung.com>