In __wt_find_lookaside_upd, when we encounter a modify, we read backwards until we find a base update and then apply each modify on top of it. There is a case where the base update is not in the lookaside (the birthmark record for that key) so this function needs to be aware of that.

This ticket is to make the __wt_find_lookaside_upd function aware of the fact that the birthmark record may be the base update and to write a test workload that exercises this logic.