Fix for potential case of falling into infinite loop when pop queue

Fix for potential case of falling into infinite loop when pop queue

TC-2402

if backend's process takes very long times, the client who requests
something to server will get timeout because the server cannot receive
the request from client because it is busy for looping on queue_job

when received dbus request, add idler to handle it at the next event
loop iteration
(replace direct call of queue_job with add an idler)
and add again when backend exit to handle next queued request

remove unwanted local variable initializing

Change-Id: I854b0b90356d9d875951b053e2c090eae1192a6b
Signed-off-by: Sangyoon Jang <s89.jang@samsung.com>