[patch] Improved duplicate suppression

[Kristóf Katus]

Hi all, 

Currently the duplicate suppression mechanism of cyrus-imapd is only based on 
the Message-Id and the To field of the email header. Some email clients reuse 
the Message-Id if you resend the same email later on, so the resent email is 
going to be suppressed by cyrus. 

The solution for this is to not only store the message-id/mailbox pair in the 
duplicate delivery database, but also the Date field of the given email (so the  
message-id/mailbox/date triple is going to be stored). You can find a patch 
attached, which applies to cyrus-imapd-2.4.10 and solves the problem. This is 
my first open source contribution, please review my code and tell how I should 
proceed. 

I would like to also add, that the altered code paths are all tested (all the 
modified functions are called at least once during the tests), and a patched 
cyrus-imapd is currently running on a productive mail server. 

Best regards, 

Kristóf Katus
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