validate res_iter before writing to lfcgi.stdout

[rafis]

[Feature request] Add new parameter t.validate_before_output or t.one_chunk_output or t.complete_output_only, where t is second parameter of _M.run(). If the parameter set and equal to true this will run additional step of res_iter validation storing output in temporary table before actual sending output to lfcgi.stdout. After validation succeeded send stored table using table.concat(). If validation failed then call send_error().

This addition will prevent cases there half of page being outputted and then ======== WSAPI ERROR DURING RESPONSE PROCESSING:.

Or maybe this can be considered as PHP ob_start()/ob_end()/ob_flush() feature.