[req] Pull all mail instead of just un-seen

[dekimsey]

IMAP is intended to be used by multiple mail clients. We have some ops/qa folk that check the mailboxes independently of logstash. Their viewing is marking the messages as seen and the plugin is hard-coded to skip those messages. I'd like for the plugin to pull all messages instead of just UNSEEN ones.

To do this, the plugin would need to use the UID imap api and track the current uid index similar to how logstash-input-file tracks file pointers. IMAP UIDs have some uniqueness guarantees that I feel can be leveraged here.

There is something to be said for also setting the fingerprint on the event to a hash of (folder name, folder UIDVALIDITY, message UID) tuple.

• Version: logstash-5.3.2
• Operating System: CentOS 7

[pemontto]

Marking as read issue fixed in #46