Honor run_as_owner option in tablesync worker.

 Honor run_as_owner option in tablesync worker.
 
 Commit 482675987 introduced "run_as_owner" subscription option so that
 subscription runs with either the permissions of the subscription
 owner or the permission of the table owner. However, tablesync workers
 did not use this option for the initial data copy.
 
 With this change, tablesync workers run with appropriate permissions
 based on "run_as_owner" option.
 
 Ajin Cherian, with changes and regression tests added by me.
 
 Reported-By: Amit Kapila
 Author: Ajin Cherian, Masahiko Sawada
 Reviewed-by: Ajin Cherian, Amit Kapila
 Discussion: https://postgr.es/m/CAA4eK1L=qzRHPEn+qeMoKQGFBzqGoLBzt_ov0A89iFFiut+ppA@mail.gmail.com