Flushes the process-queue, but doesn't exit. This simply drains the queue
and ensures all worker threads have finished their current tasks
associated with this process.
NOT: This does not mean the worker threads are not executing jobs in
another process-queue.
Flushes the process-queue, but doesn't exit. This simply drains the queue and ensures all worker threads have finished their current tasks associated with this process.
NOT: This does not mean the worker threads are not executing jobs in another process-queue.
Returns 0 on success; -1 on failure