Use mail_map copies for all complex ops

* src/mail.cpp (Mail::revertAllMails_o, Mail::wipeAllMails_o),
(Mail::closeAllMails_o): Use copy of the mail map.

For any complex operation we can't hold the lock of the
map for the operation as the map might be modified / accessed
from a different thread to complete the operation.

It should not be a big problem with these functions.