Improvements in Waitable Queue

[sgn4sangar]

The waitable_queue.hpp has a bit of scope to improve

1. It is better not to expose the underlying queue through following queue functions
   Queue& queue() noexcept { return queue_; } const Queue& queue() const noexcept { return queue_; }

2. As of now the epoll system calls and the queue adapter APIs can be accessed independently
   Eg: try_dequeue(Output& value) removes an item but does not drain the eventfd. If a consumer calls try_dequeue() without first calling drain_notifications().., epoll_wait() may continue reporting the queue fd has still data.
   The order of calling the functions shall be, somehow, enforced appropriately; like epoll_wait() followed by drain_notifications() and dequeue