jaba discovery hangs if URL leads to non-responding server

If Jalview tries to discover services at a on-responding URL, it will hang for the default Java timeout of 300s for each non-responding server - so the timeout needs to be reduced for the JWS client.

Furthermore: Jaba WS discovery is currently done by a singleton thread that serially processes the URL list. If a server is non-responsive, it will block the discovery of other services, and if the user restarts the discovery process, Jalview will not cancel the current discovery thread and start a new one.

Service discovery needs to be series-parallel, and process needs to be cancellable - by the user or by Jalview if the set of discovery urls are modified.