Add when_all to our coroutine library

[marty1885]

This PR implements when_all. A new function to wait for multiple coroutines to finish. This function is designed to not suffer from head of line blocking when a coroutine takes particularly long to finish.

Before this PR, we write the following. Which forces the next request to be sent after reeving response from the server.

for(int i=0;i<10;i++)
{
    co_await client->sendRequestCoro(req);
}

With this PR, we can do the following, Which will queue all request into the client before waiting for all responses.

std::vector<Task<>> tasks;
for(int i=0;i<10;i++)
{
    tasks.push_back(client->sendRequestCoro(req));
}

co_await when_all(std::move(tasks));