Make async_mutex::lock_async behaviour more explicit by introducing a scoped_lock_async() method

[lewissbaker]

The async_mutex::lock_async() method currently has the dual behaviour of allowing you to lock the mutex in such a way that requires manual unlocking (ie. no RAII) as well as allowing you to encapsulate the lock in an async_mutex_lock object that will ensure the lock is released when the lock object goes out of scope.

The behaviour is currently chosen based on whether the caller assigns the result of the co_await m.lock_async() expression to an async_mutex_lock variable.

task<T> f(async_mutex& m)
{
  // Needs manual unlocking
  co_await m.lock_async(); 
  m.unlock();
}

task<T> g(async_mutex& m)
{
  // Lock encapsulated in RAII object
  async_mutex_lock lock = co_await m.lock_async(); 
}

This seems potentially error-prone way of doing things.

Consider having lock_async() always require manual unlocking and introduce a new scoped_lock_async() that returns an async_mutex_lock object.

[lewissbaker]

Addressed in 1140628