Fixes COMETD-468 (Improve reconnection behavior for WebSocket).

Now the WebSocket transport remembers if it was able to connect
and does not disable the transport if it was.
BayeuxClient.onFailure() now logs at debug level, so there is no
need to override it to suppress exception stack traces in tests.
Fixed tests that was failing due to the new semantic of disconnect.
Now the WebSocket transport closes the connection as soon as the
disconnect response is processed on the client (before it was leaking
connections).