Make MailFolder faster and safer

MaiBox folder seems to have some performance issues :

• there is no locking to avoid multiple threads to fetch emails at the same time
• the default imap configuration was done to avoid bugs on some servers but may not be optimum (read mails several times)

For now, let's add some system properties to be able to tweak the behavior :

// Force IMAP lib to do a Copy of the message : default is now false
(Seetings is supposed to workaround some IMAP bugs ...)

org.nuxeo.mail.imap.copy = false

// debug info : default is now false
(debug now false by default, but we keep a system property to activate it)

org.nuxeo.mail.imap.debug = true

Locking is now managed at 2 levels :

• inside the scheduled listener : avoid re-entrency
• inside the MailCoreHelper : avoid 2 threads to fetch the same mailbox