[TRANSPORT] never send requests after transport service is stopped

With local transport or any transport that doesn't necessarily send
notification if connections are closed we might miss a node
disconnection and the request handler hangs forever / until the timeout
kicks in. This window only exists during shutdown and is likely
unproblematic in practice but tests might run into this problem when
local transport is used.
This can cause double exception handling, one on the caller thread and one on the handler (if the transport is closed between the two lines).  I think we should rely on the clientHandels (concurrent map) and rather then throw an exception do the same logic we have in the finally clause in  doStop - removed the holder from the clientHandlers, if that works out, call the handler with an exception. Makes sense?