Move some logic that is currently in ATExecAttachPartition to
separate functions to facilitate future code reuse.

Ashutosh Bapat and Jeevan Ladhe

Discussion: http://postgr.es/m/CA+Tgmobbnamyvii0pRdg9pp_jLHSUvq7u5SiRrVV0tEFFU58Tg@mail.gmail.com