Make support for unconnected clients optional

[GregDThomas]

[Duplicate of Issue 15 from the old code.google.com site - http://code.google.com/p/aspcomet/issues/detail?id=15]

The spec @ http://svn.cometd.org/trunk/bayeux/bayeux.html#toc_65 suggests that it's optional for the server to accept a publish without a client id ("It is OPTIONAL for a server to accept unconnected publish requests").

Currently AspComet will publish messages from unconnected clients - and it's possible to prevent that by catching each PublishingEvent and cancelling those without a valid client id.

However, adding a subscription for each PublishingEvent just to check the client id could be a non-trivial overhead. For that reason, the support for unconnected clients should be configurable, e.g. a new property, allowUnconnectedClients, default=true.