Thread: merge from trunk last updates since 1.54.

[SVN r85200]
Thread: merge from trunk upgrade_to_unique_lock::mutex() + doc typos + BOOST_THREAD_USES_ATOMIC doc + synchronized _value call + rename wait_until params.

[SVN r85603]
Thread: fix #8417.

[SVN r85621]
Thread fix #8943.

[SVN r85622]
Thread: merge scoped_thread constr + condition_variable timed wait issues on windows + doc typos.

[SVN r85663]
Thread: merge atomic linked conditionaly + future:get_exception_ptr.

[SVN r85666]
Thread: merge from trunk to fix 8070 and possibly 7461.

[SVN r85815]
Merged changes from trunk.

[SVN r85818]
Thread: fix non-copyable issues with intel compiler.

[SVN r85842]
 Thread: add a test on get_exception_ptr.

[SVN r85843]
Thread: ensure that the thread is joined on the future destructor created by future<T>::then().

[SVN r86010]
Thread: fix some issues with double definition of shared_state_base and avoid ambiguity of disable_if with Boost.Test.

[SVN r86202]