Whole table is rebuilt on node save

Issue Summary

similarnodes_build_cache() wipes out the whole of the similarnodes table and rebuilds it from scratch on every cron run and node save. This is quite probably a performance hit on large sites.
There's a setting for whether to rebuild on node save, but then that still leaves a cron run that rebuilds the whole table.

Wouldn't it be more efficient to drop only the saved node's rows on node save?
With cron though IIRC there's an issue around modules like community tags that change node terms without saving the node -- not sure how that would be best handled.