Do not use mail library to process mails via IMAP.

Because it is too slow to receive mails with mail library.
We need to process a lot of mails in this project.

For example receive over 3000 mails,

 mail:     not finish in 60 seconds
 net/imap: finish in 30 seconds
Add CommitCommentTools::MailCounter.

But not implemented to generate CSV.