- refactor ! MapperProperty now has create_row_processor() method, returns

- refactor !  MapperProperty now has create_row_processor() method, returns
a tuple of (rowprocessor, post_processor) functions.  this is called
once per mapper per result set, given just one row (usually the first row)
in the result set, so that all MapperProperty objects can make decisions based on that row
which get saved for all successive rows.
essentially it allows lots of decision making and pre-calculations to happen per-result
instead of per-row, and decreases how much state needs to be placed in SelectContext.attributes
since the functions are closures.