mapper - pks_by_table should store keys in order even tho we dont have a failure...

mapper - pks_by_table should store keys in order even tho we dont have a failure case
lazyloader can lazyload using mapper.get() if it is appropriate, saves a lot of queries
a few more assertions in properties in prep for relations against select statement mappers
mapper get() clause is determined upfront to avoid re-generating it