[k2] implement wait_concurrently

[apolyakov]

This pull request introduces wait_concurrently by adding a new coroutine type called shared_task_t. This new type will replace task_t in all contexts where forks are utilized.

The shared_task_t coroutine type allows multiple co_await operations from different parts of the code. It keeps track of all the awaiters that need to be resumed once it completes. Like task_t, shared_task_t also supports cancellations.