Prevent duplicate detach() calls with push (#13261)

This used to happen when push was disconnected due to a UI or session
expiring. requestStart() and requestEnd() were called on disconnect
even though a disconnection is not a request.

Change-Id: I31d9cae65ec75b5046802a54bbe4564d6e44b29f
Merge commit '3d0ff32bea81c3e3c64bd044276ff04a4f8555ed'

Conflicts:
	server/src/com/vaadin/server/communication/AtmospherePushConnection.java
	server/src/com/vaadin/server/communication/PushHandler.java

Change-Id: I93b5c3f3017aaa1ba2a231028c808d6e56fe1c11