Added checks to verify that we do not try and clear the foreverFrame iframe DOM when we don't need to.

- This involved adding a state check for connected and deleting the frame message count on transport stop.
- Also added an iframeClearThreshold value to the forever frame transport to allow me to test it.

#2249
Added tests to verify that forever frame does not throw in start/stop stress scenarios.

#2249
Moved the connection state check outside of the frame message count set.

- This protects against us accidently setting a bad frameMessageCount value after the connection has been stopped.
Merge branch 'release' into dev

Conflicts:
	samples/Microsoft.AspNet.SignalR.Samples/Scripts/jquery.signalR.min.js