[PATCH] pgarchives: parser: handle messages in which Message-ID is missing

Hello,

As surprising as it may seem, Message-ID is actually not a mandatory email field [1]. While most MTAs do add this
field,some might not, and this will cause load_message.py to crash.
 
As a solution to this, when this field is missing, this patch:
- attempts to find a "Sent-Message-ID" header and use it as the Message-ID (a case I encountered when trying to import
anold mbox)
 
- generates a new Message-ID if none exists, following (a simpler version of) [2].

[1] https://www.rfc-editor.org/rfc/rfc2822#section-3.6.4
[2] https://datatracker.ietf.org/doc/html/draft-ietf-usefor-message-id-00#section-3

Cheers,
-- 
Célestin Matte