[FaceRest] Add a comment about life-span of libsoup's objects
[FaceRest] Add a skeleton of "Worker" class.

FaceRest::Worker will process REST jobs in parallel.
[FaceRest] Add a queue for RestJob
[FaceRest::Worker] Add a tentative implementation of mainThread()
[FaceRest] Add tentative functions to start & stop workers
[FaceRest] Check existence of queue elements before calling front().

Calling front() with empty queue is undefined.
[FaceRest] Add a semaphore to wait queuing next job
[FaceRest] Reorder member variables in PrivateContext
[FaceRest] Add a flag to switch sync/async mode
[FaceRest] Implement stopping worker threads
[FaceRest] Delete worker objects on stopWorkers()
[FaceRest] Use pre-increment of an iterator to improve performance
[FaceRest] Output a log message on exiting a worker thread
[FaceRest] Remove needless stop() call for Worker
[FaceRest] Fix a bug that RestJob::unpauseResponse() is called twice when handlerGetItem is called synchronously
[FaceRest] Enable async mode by default
[FaceRest] Unify duplicated code
[FaceRest] Add some comments
[FaceRest] Output a log message on starting a worker
[FaceRest] Fix brace positions