Fix possible "tuple concurrently updated" error in ALTER TABLE.

When adding an inheritance parent to a table, an AccessShareLock on the
parent isn't strong enough to prevent trouble, so take
ShareUpdateExclusiveLock instead.  Since this is a behavior change,
albeit a fairly unobtrusive one, and since we have only one report
from the field, no back-patch.

Report by Jon Nelson, analysis by Alvaro Herrera, fix by me.