Increase test timeout since SafeCTS.Cancel() can slow down execution

- Running tests in parallel led to frequent timeouts here
Enumerate messages before completing the Subscribe callback in tests

- Subscription.Work was previously clearing the list during enumeration
Simplifying ServerSentEventsTransport
 - NotSafeForWork - Removing unnecessary SafeInvoker from ServerSentEventsTransport
 - DisposingOfDisposer - unifying request cancellation
FoundABetterHome - moving ProcessResponse to ClientTransportBase class
HowToDismantleALongPollingBomb#1 - merging LongPolingTransport and PollingRequestHandler
HowToDismantleALongPollingBomb#2 - removing NegotiateInitializer
HowToDismantleALongPollingBomb#3 - Separting connect request from polling loop
HowToDismantleALongPollingBomb#4 - Merging connect back to long polling loop

Turned out that the init ("S":1) message does not have to be the response to the "connect" request and therefore for the client the "connect" request is not conceptually different from the "poll" request. As a result they should be handled the same way and should both be part of the long polling loop.