Disable keep alive checks in long polling transport

#3259
Unfortunately, the semantics of `ConnectionTimeout` is kind of confusing, but it can probably be best under stood as a long poll timeout. I believe that is the only way our customers use it.

In SignalR 3.0, we've already moved this configuration to `SignalROptions.Transports.LongPolling.PollTimeout` which I think is much more clear.

`ConnectionTimeout` can be a bit confusing is because [some of our tests](https://github.com/SignalR/SignalR/blob/2.1.1/tests/Microsoft.AspNet.SignalR.FunctionalTests/Client/ConnectionFacts.cs#L429) use it to force reconnects on *all* transports. In our tests, we force other transports to use the `ConnectionTimeout` by setting `KeepAlive` to null.

The reason we are now sending the `ConnectionTimeout` to the client is so that the client can know whether or not the server failed to close the connection after the timeout. If this happens, it is likely due to the network going down, so the client should attempt to reconnect.

We decided this would be nice to have since the client will no longer attempt to reconnect if it misses keep alives over long polling.