Couchstore compactor output _must_ be logged

I've spotted occasional: "Couchstore error: checksum error" on terminal output. But I was unable (!!!!) to find this in logs. Later I realized that we're spawning so called native compactor in a way that's spilling output to erlang's stderr. Basically all output from this is lost.

Also I spotted mis-reviewed logging in couch_db_updater:start_copy_compact. Failures of native compactor are logged with loglevel debug (!!!). Which means they are discarded as well.