leave op metadata on deleted documents for later querying

we want to have a bit more knowledge about deleted snapshots without having to dig in the oplog. This will help find recently deleted documents, as well as be able to query by other metadata (like accountId, userId) for cleaning up lapses in indexing.
If users don't want to keep the metadata around, deleting data.m in the validate step should let them.

I added a opData.validate function in our middleware that copies op metadata into the snapshot, so our defualt behavior is to store more information