Possibly wrong readers count in boost::thread_v2::upgrade_mutex

Because of issues (https://github.com/boostorg/thread/issues/265, https://github.com/boostorg/thread/issues/361) in the default shared mutex implementation, I reviewed the code of the V2 mutex classes.

It seems like `upgrade_mutex::try_unlock_shared_and_lock_until(const boost::chrono::time_point& abs_time)` and `upgrade_mutex::try_unlock_upgrade_and_lock_until(const boost::chrono::time_point& abs_time)` can set the readers count to a wrong value.

Both functions contain the same piece of code:
```
count_t num_readers = (state_ & n_readers_) - 1;
state_ &= ~n_readers_;
state_ |= (write_entered_ | num_readers);
if (!gate2_.wait_until(lk, abs_time, boost::bind(
&upgrade_mutex::no_readers, boost::ref(*this))))
{
++num_readers;
state_ &= ~(write_entered_ | n_readers_);
state_ |= num_readers;
```

The problem is with `++num_readers`. Between reading it out of the state and then setting it again, the mutex is unlocked, so readers can `unlock_shared()`. This is even necessary, as the code actually waits until the readers count reaches 0.

So IMHO, the line in question should read:
```
num_readers = (state_ & n_readers_) +1;
```