Fix performance counter hanging issues.
- Always try to create the SignalR performance counters without checking
  for existence since that hangs.

#1158
Removed unused variable.
Fix issue on .NET 4.0 where response cancellation token is already disposed.
- Dispose the token only if it's not a result of ending an existing
  connection.
- Dispose the token before ending the connection.
- Catch ObjectDisposedException in SafeRegister's DisposableAction.

#1549
Merge branch 'release' into dev

Conflicts:
	src/Microsoft.AspNet.SignalR.Core/Infrastructure/CancellationTokenExtensions.cs
	src/Microsoft.AspNet.SignalR.Core/Transports/TransportHeartBeat.cs
Fix issue on .NET 4.0 where cancellation token is already disposed.
Ported 8e3c0e79f24c9fdbaf13864c7914534c2d059c11 to dev.