pthreadpool: add some lockless coordination between the main and job threads

pthreadpool: add some lockless coordination between the main and job threads

In the direction from the main process to the job thread, we have:

- 'maycancel', which is set when tevent_req_cancel() is called,
- 'orphaned' is the job request, tevent_context or pthreadpool_tevent
  was talloc_free'ed.

The job function can consume these by using:

   /*
    * return true - if tevent_req_cancel() was called.
    */
   bool pthreadpool_tevent_current_job_canceled(void);

   /*
    * return true - if talloc_free() was called on the job request,
    * tevent_context or pthreadpool_tevent.
    */
   bool pthreadpool_tevent_current_job_orphaned(void);

   /*
    * return true if canceled and orphaned are both false.
    */
   bool pthreadpool_tevent_current_job_continue(void);

In the other direction we remember the following points
in the job execution:

- 'started'  - set when the job is picked up by a worker thread
- 'executed' - set once the job function returned.
- 'finished' - set when pthreadpool_tevent_job_signal() is entered
- 'dropped'  - set when pthreadpool_tevent_job_signal() leaves with orphaned
- 'signaled' - set when pthreadpool_tevent_job_signal() leaves normal

There're only one side writing each element,
either the main process or the job thread.

This means we can do the coordination with a full memory
barrier using atomic_thread_fence(memory_order_seq_cst).
lib/replace provides fallbacks if C11 stdatomic.h is not available.

A real pthreadpool requires pthread and atomic_thread_fence() (or an
replacement) to be available, otherwise we only have pthreadpool_sync.c.
But this should not make a real difference, as at least
__sync_synchronize() is availabe since 2005 in gcc.
We also require __thread which is available since 2002.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>