view: stop storing all MIME objects on large threads

While we try to discard the $smsg (SearchMsg) objects quickly,
they remain referenced via $node (SearchThread::Msg) objects,
which are stored forever in $ctx->{mapping} to cull redundant
words out of subjects in the thread skeleton.

This significantly cuts memory bloat with large search results
with '&x=t'.  Now, the search results overhead of
SearchThread::Msg and linked objects are stable at around 350K
instead of ~7M per response in a rough test (there's more
savings to be had in the same areas).

Several hundred kilobytes is still huge and a large per-client
cost; but it's far better than MEGABYTES per-client.

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