Lookaside optimization: birthmarks

If we had "birthmarks", an inverse of tombstones (WT_UPDATE with type WT_UPDATE_DELETED), with the following sematics:

• if a birthmark is not visible, it is equivalent to a tombstone; and
• if a birthmark is visible, the on-page value is returned.

Then when saving updates for lookaside eviction or update / restore, we could create a birthmark in the saved update list instead of making a full copy of the on-page value. For many common cases, this could dramatically reduce the amount of data that has to be written to the lookaside table (or stored in cache).