lmtpengine: only use the message-id header for deduplication

https://tools.ietf.org/html/rfc5322 section 3.6.6

[...] Resent fields are
strictly informational.  They MUST NOT be used in the normal
processing of replies or other such automatic actions on messages.

And we have discovered that using x-me-message-id sometimes discards
emails that we actually care about