[PATCH 2/3] lib-storage: thread - Limit References: header msgid count to prevent...

[PATCH 2/3] lib-storage: thread - Limit References: header msgid count to prevent O(N^2) CPU usage

Cap per-message References ingestion at MAIL_THREAD_REFERENCES_MAX (1000)
in mail_thread_map_add_mail(). Without this limit a single crafted email
with N unique Message-IDs triggers N(N-1)/2 ancestor traversals in
thread_node_has_ancestor(), allowing unauthenticated DoS via mail delivery.

Gbp-Pq: Name 0002-lib-storage-thread-Limit-References-header-msgid-cou.patch