When IMAP variables were changed during a reload, Voicemail did not use the new values.

When IMAP variables were changed during a reload, Voicemail did not use the new values.
This change introduces a configuration version variable, which ensures that
connections with the old values are not reused but are allowed to expire
normally.
(closes issue #15934)
 Reported by: viniciusfontes
 Patches:
       20090922__issue15934.diff.txt uploaded by tilghman (license 14)
 Tested by: viniciusfontes

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@219816 65c4cc65-6c06-0410-ace0-fbb531ad65f3