[req] Use the IDLE IMAP api instead of polling

[dekimsey]

The current implementation polls the IMAP server for new mail re-creating the connection with every event. For faster message pulling it'd be awesome if instead the plugin used the IMAP IDLE command for supporting servers. Less connection creation/teardowns and far more immediate event processing.

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

[dekimsey]

Context: We dropped the polling interval and ran into the IMAP server rejecting connections because they were a wee bit too frequent.