Improve tracking of connection lifetime

[halter73]

I haven't seen this cause any issues in practice, but the following sample shows why the way we're currently tracking connection lifetimes is at least theoretically bad. I know we also want to start tracking non-HTTP connections. This PR doesn't do that, but I think we should still take this change in the meantime.

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace ConnectionManagerRace
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var manager = new ConnectionManager();
            Task.Run(() => StartConnections(100, manager));
            Thread.Sleep(500);
            manager.CloseAllConnectionsAsync().Wait();
        }

        public static void StartConnections(int count, ConnectionManager manager)
        {
            for (var i = 0; i < count; i++)
            {
                var conn = new Connection(manager);
                conn.StartRequestProcessing();
            }
        }
    }

    public class ConnectionManager
    {
        private readonly ConcurrentDictionary<long, Connection> _connections = new ConcurrentDictionary<long, Connection>();
        private long _lastId = 0;

        public void Add(Connection conn)
        {
            _connections.TryAdd(Interlocked.Increment(ref _lastId), conn);
        }

        public async Task CloseAllConnectionsAsync()
        {
            var closeTasks = new List<Task>();

            foreach (var pair in _connections)
            {
                closeTasks.Add(pair.Value.StopProcessingNextRequestAsync());
            }

            await Task.WhenAll(closeTasks);
        }
    }

    public class Connection
    {
        private readonly ConnectionManager _manager;
        private Task _lifetimeTask;

        public Connection(ConnectionManager manager)
        {
            _manager = manager;
        }

        public Task StartRequestProcessing()
        {
            return _lifetimeTask = ProcessRequestsAsync();
        }

        public async Task ProcessRequestsAsync()
        {
            _manager.Add(this);
            Thread.Sleep(1000);
            await Task.Delay(1000);
        }

        public Task StopProcessingNextRequestAsync()
        {
            return _lifetimeTask;
        }
    }
}

Which results in:

ConnectionManagerRace> dotnet run

Unhandled Exception: System.AggregateException: One or more errors occurred. (The tasks argument included a null value.
Parameter name: tasks) ---> System.ArgumentException: The tasks argument included a null value.
Parameter name: tasks
   at System.Threading.Tasks.Task.WhenAll(IEnumerable`1 tasks)
   at ConnectionManagerRace.ConnectionManager.CloseAllConnectionsAsync() in G:\dev\halter73\ConnectionManagerRace\Program.cs:line 48
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at ConnectionManagerRace.Program.Main(String[] args) in G:\dev\halter73\ConnectionManagerRace\Program.cs:line 16

[halter73]

@davidfowl ping

[davidfowl]

LGTM for now. We need to write up our design notes from the last discussion.