Make timestamp usage checking at commit the default

If you commit twice to the same key such that the commit timestamps are in order, but the durable timestamps are reversed, we assert in reconciliation when moving the first value to history.

Since we think out of order commits are going away entirely, with luck this is a nonissue.

However, the in-order timestamp assertions probably ought to be checking up front that durable timestamps on individual items don't go backwards; evidently that's not currently the case.

Will attach a simple reproducer.