Message threading

Message should be grouped into threads based on In-Reply-To, References Message-Id header fields. Message can belong to one or more threads. Storage layer should be able to produce thread tree (each message is linked directly to the message it is replying to) or flattened thread list (just list every message in thread).

Since each message belong to at least one thread and most threads don't have more than one message, we don't want to create a separate thread object for each message. Consider reusing message object as a thread object - each message contains two references - thread root message ID and thread parent message ID.

maddy-storage should support two threading algorithms (ORDEREDSUBJECT and REFERENCES) described in https://datatracker.ietf.org/doc/html/rfc5256. Note that go-imap v2 does not support THREAD extension server-side yet: https://github.com/emersion/go-imap/issues/533

For future JMAP compatibility, we don't merge threads if messages arrive out-of-order as this would require changing message IDs.

Beginne damit, den Kontext des storage backend und die Abschnitte von RFC 5256 zu lesen, in denen ORDEREDSUBJECT- und REFERENCES-Threading beschrieben werden. Identifiziere die beteiligten Message- und Storage-Einstiegspunkte und lege anschließend fest, wie Thread-Bäume und abgeflachte Listen erzeugt werden sollen, wobei die message IDs erhalten bleiben müssen, wenn Nachrichten in ungeordneter Reihenfolge eintreffen.

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