Make Received: header special in notmuch_message_file_get_header

With this patch the Received: header becomes special in the way
we treat headers - this is the only header for which we concatenate
all the instances we find (instead of just returning the first one).

This will be used in the From guessing code for replies as we need to
be able to walk ALL of the Received: headers in a message to have a
good chance to guess which mailbox this email was delivered to.

Signed-off-by: Dirk Hohndel <hohndel@infradead.org>