example future_when_all deadlock between log mutex and future wait

Hi. I ran the `future_when_all.cpp` example with `BOOST_THREAD_USES_LOG` on. And see the following deadlock:

```
# pstack `pidof boost-thread`
Thread 2 (Thread 0x7fefed7406c0 (LWP 58475) "boost-thread"):
#0 0x00007fefee7caf50 in __lll_lock_wait () from /lib64/libc.so.6
#1 0x00007fefee7d146a in pthread_mutex_lock@@GLIBC_2.2.5 () from /lib64/libc.so.6
#2 0x00000000004110a8 in boost::posix::pthread_mutex_lock (m=0x451500 ) at /data/opt/boost/include/boost/thread/pthread/pthread_helpers.hpp:79
#3 boost::recursive_mutex::lock (this=0x451500 ) at /data/opt/boost/include/boost/thread/pthread/recursive_mutex.hpp:108
#4 0x0000000000413e18 in boost::lock_guard::lock_guard (this=0x7fefed73fc78, m_=...) at /data/opt/boost/include/boost/thread/lock_guard.hpp:38
#5 0x00000000004067ba in p1b () at /mnt/cxx/boost/thread-example/future_when_all.cpp:41
#6 0x000000000042758e in boost::detail::invoke (f=@0x151f558: 0x40679a ) at /data/opt/boost/include/boost/thread/detail/invoke.hpp:140
#7 0x000000000042530c in boost::detail::invoker::execute<>(boost::detail::tuple_indices<>) (this=0x151f558) at /data/opt/boost/include/boost/thread/detail/invoker.hpp:102
#8 0x0000000000422f08 in boost::detail::invoker::operator() (this=0x151f558) at /data/opt/boost/include/boost/thread/detail/invoker.hpp:95
#9 0x00000000004204c1 in boost::detail::future_async_shared_state >::run (that=..., f=...) at /data/opt/boost/include/boost/thread/future.hpp:981
#10 0x000000000042f003 in boost::detail::invoke > >, boost::detail::invoker&&), boost::shared_ptr > >, boost::detail::invoker > (f=@0x151f570: 0x420495 >::run(boost::shared_ptr > >, boost::detail::invoker&&)>) at /data/opt/boost/include/boost/thread/detail/invoke.hpp:140
#11 0x000000000042e9cd in boost::detail::thread_data > >, boost::detail::invoker&&), boost::shared_ptr > >, boost::detail::invoker >::run2<1ul, 2ul> (this=0x151f420) at /data/opt/boost/include/boost/thread/detail/thread.hpp:79
#12 0x000000000042c126 in boost::detail::thread_data > >, boost::detail::invoker&&), boost::shared_ptr > >, boost::detail::invoker >::run (this=0x151f420) at /data/opt/boost/include/boost/thread/detail/thread.hpp:85
#13 0x00007fefeed5d3e0 in thread_proxy () from /data/opt/boost/lib/libboost_thread.so.1.90.0
#14 0x00007fefee7ce12d in start_thread () from /lib64/libc.so.6
#15 0x00007fefee84fbc0 in clone3 () from /lib64/libc.so.6

Thread 1 (Thread 0x7fefeeca7000 (LWP 58458) "boost-thread"):
#0 0x00007fefee7cad16 in __futex_abstimed_wait_common () from /lib64/libc.so.6
#1 0x00007fefee7cd510 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#2 0x000000000040e51d in boost::posix::pthread_cond_wait (m=0x151ef18, c=0x151ef40) at /data/opt/boost/include/boost/thread/pthread/pthread_helpers.hpp:112
#3 boost::condition_variable::wait (this=0x151ef18, m=...) at /data/opt/boost/include/boost/thread/pthread/condition_variable.hpp:79
#4 0x0000000000413239 in boost::condition_variable::wait, boost::_bi::list > > > (this=0x151ef18, m=..., pred=...) at /data/opt/boost/include/boost/thread/pthread/condition_variable_fwd.hpp:93
#5 0x00000000004102d9 in boost::detail::shared_state_base::wait_internal (this=0x151eec0, lk=..., rethrow=true) at /data/opt/boost/include/boost/thread/future.hpp:395
#6 0x000000000041c74e in boost::detail::shared_state::get_storage (this=0x151eec0, lk=...) at /data/opt/boost/include/boost/thread/future.hpp:647
#7 0x0000000000418177 in boost::detail::shared_state::get (this=0x151eec0, lk=...) at /data/opt/boost/include/boost/thread/future.hpp:652
#8 0x00000000004140fe in boost::future::get (this=0x7ffc9ee4dda0) at /data/opt/boost/include/boost/thread/future.hpp:1804
#9 0x0000000000409103 in t_future_when_all () at /mnt/cxx/boost/thread-example/future_when_all.cpp:238
#10 0x000000000040668c in main (argc=2, argv=0x7ffc9ee4e758) at /mnt/cxx/boost/thread-example/thread_example.cc:6

```

Maybe we should split the operation:
> BOOST_THREAD_LOG << res[1].get()

into two separate statements:
> auto i = res[1].get();
BOOST_THREAD_LOG << i