Close more consistently

[Minimaximize]

Ensure that calling close does not result in orphan tasks which will never be completed.

Previously each call of Close(details, reason) would result in the mCloseTask field in WampSessionClient being overwritten with a new task completion source, meaning previous calls to Close could no longer be completed as the session client no longer had access to the underlying TaskCompletionSource

Changes

• Calling Close before the session client has initiated the wamp session (by sending a Hello to the server) throws a WampSessionNotEstablishedException
• Calling Close before the SessionClient has connected (i.e. during a Challenge or before a Welcome message has been issued) will instead call Abort
• Calling Close multiple times before the channel disconnects will return the same task

[Minimaximize]

It occurred to me as I was making the pull request that I may have been aborting the connection to the server incorrectly (instead calling Abort as though the message had come from the server)

On that note, There is an issue where the WampSessionServer expects the Realm of a WampClientProxy to have already been set prior to Abort being called