Apply changed from #9471 Synchronization documentation nits and prepare changes notes for Boost 1.56

	modified:   changes.qbk
	modified:   external_locking.qbk
	modified:   internal_locking.qbk
	modified:   thread.qbk
Update sync queues closed operations
Fix #9335 Missing exception safety might result in crash
Reorganized executors on its own directory adding serial_executor and renamed thread_pool by basic_thread_pool and user_scheduler by loop_scheduler.
fix typo on try_push_back.
executor_adaptor<serial_executor> a(b) doesn't compiles on C++03.
Added missing iomanip include.
Make work public.
Make work public.
Added inline and thread executors.
Added minimal inline and thread test.
Added at thread entry callback on thread pool.
cleanup comments.
Fix issues with future::async and disable the code when BOOST_NO_CXX11_DECLTYPE_N3276 is defined as it is bugged.
reorder async() conditional compilation variants to factor more code.
indent and comment #if.
Try to see if giving the addres solves the MSVC compiler issue.
make shared_state_nullary_task copyable and movable and avoid msvc default generation.
Update mutex destructor to see if the assertion message is better.
Try to solve some issues with invoke on MSVC.