MailHost has wrong strategy creating queue processing threads

[Maric Michaud]

I was surprised by the behavior of asynchronous mailing in zope 2.11.3
MailHost product. It creates a thread by instance of MailHost by the
mean of using self.absolute_url(1) as key for the queue_threads dictionnary.

It seems obviously wrong, IMO, if several instances share the same queue directory,
mesages actually get sent more than once.

Following is a patch which works fine, but the idea is
straightforward, it's to use the realpath of the smtp_queue_directory as
the identifier of the thread, ensuring that MailHosts that share the
same queue directory will also share the same thread for processing mails.

This bug apply to zope 2.12 too, but the provided patch should work the same way.

[Tres Seaver (tseaver)]

Thank you very much for the patch! Now committed to the 2.11 branch:

 http://svn.zope.org/Zope/branches/2.11/?rev=111898&view=rev

the 2.12 branch:

 http://svn.zope.org/Zope/branches/2.12/?rev=111900&view=rev

and the trunk:

 http://svn.zope.org/Zope/trunk/?rev=111901&view=rev