New Rmail summary "by thread"

New Rmail summary "by thread"

* lisp/mail/rmailsum.el (rmail-summary-subjects-hash-table)
(rmail-summary-message-parents-vector)
(rmail-summary-message-ids-hash-table): New variables.
(rmail-summary-fill-message-ids-hash-table)
(rmail-summary--split-header-field)
(rmail-summary-fill-message-parents-vector)
(rmail-summary-direct-descendants)
(rmail-summary--walk-thread-message-recursively)
(rmail-summary-by-thread): New functions.

* etc/NEWS: Announce the new Rmail features.