www: deduplicate Message-ID in threading + skeleton HEAD master

xt/perf-threading.t reports a small 0.5-1.0% memory reduction in
non-ancient Perls with CoW strings for threading alone (w/o
rendering the View.pm stuff).

On informal tests using -httpd and giant Linux stable patch set
threads (700+ messages), this ends up being roughly 5MB saved in
/T/ rendering since we use the {mid} field again in the
$ctx->{mapping} table.  This becomes even more beneficial if
handling parallel HTTP requests for messages in the same message
thread, even across different endpoints.

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