Add SMTP ticker interval, that will limit the number of emails sent in a given period.

If more than one message is received in the period, the mails are concatenated.
Remove unneeded if statement. Always return nil after plugin is closed.
Use string slice to store queued messages (much less garbage). This also removes the need for keeping track of queue state.
Add direct sending of mails when ticker interval is 0. This will cause the mails to be sent at once and not go through a goroutine. This mode is functionally equivalent to the previous implementation.
Bit of code reorg, less []byte <-> string conversions, docs.
Merge branch 'klauspost-smtp-add-ticker-interval' into dev