Get a job from one of the job servers. This does not used the callback
interface below, which means results must be sent back to the job server
manually. It is also the responsibility of the caller to free the job once
it has been completed.
[in] worker Structure previously initialized with
gearman_worker_create() or gearman_worker_clone().
[in] job Caller allocated structure, or NULL to allocate one.
[out] ret_ptr Standard gearman return value.
On success, a pointer to the (possibly allocated) structure. On
failure this will be NULL.