(#3089) - truly auto auto-compaction

This is not done yet, but I'm already seeing huge gains in the `temp-views` test. In Chrome, the time went from 15788ms to 11976ms (24% improvement).
this should be `if (api.auto_compaction && isUpdate)`; I'll open a new PR to fix that later. Probably accounts for some of the non-speedup in leveldb.