Compact() shouldn't trigger import errors

It looks like when we call Compact() and write a "_purged":true doc, this will go through the import process and throw an error about having a _ leading property (because it's one we don't check for. This is pretty much benign, as we bail out at that point, but it would be nice if we didn't complain about it and bailed out a bit earlier.

Will try and add logs demonstrating this, but because it relies on tombstones and metadata purge, it's a pain to do a clean repro without waiting at least an hour!