config: Sleep 60 seconds if IMAP IDLE fails

* This should work on mail servers that don't have the IDLE feature.
config: Reduce IMAP IDLE keepalive to 5 minutes

* Dreamhost appears to get stuck in a state where IDLE never returns
  even though the connection appears to still be open and the log
  repeatedly prints 'S (3): * OK Still here'.  Messages arrive but
  are not handled until the daemon is restarted.
* Attempt to workaround what seems to be a IMAP server bug.