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

lisakowen/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL,snaga/postgres-xl,jmcatamney/gpdb,oberstet/postgres-xl,arcivanov/postgres-xl,50wu/gpdb,jmcatamney/gpdb,kmjungersen/PostgresXL,jmcatamney/gpdb,postmind-net/postgres-xl,ovr/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,Postgres-XL/Postgres-XL,ovr/postgres-xl,adam8157/gpdb,arcivanov/postgres-xl,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,oberstet/postgres-xl,greenplum-db/gpdb,yazun/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,xinzweb/gpdb,postmind-net/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,pavanvd/postgres-xl,adam8157/gpdb,techdragon/Postgres-XL,snaga/postgres-xl,50wu/gpdb,xinzweb/gpdb,snaga/postgres-xl,oberstet/postgres-xl,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,xinzweb/gpdb,50wu/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,50wu/gpdb,yazun/postgres-xl,kmjungersen/PostgresXL,pavanvd/postgres-xl,ovr/postgres-xl,postmind-net/postgres-xl,arcivanov/postgres-xl,greenplum-db/gpdb,50wu/gpdb,arcivanov/postgres-xl,ashwinstar/gpdb,50wu/gpdb,ovr/postgres-xl,kmjungersen/PostgresXL,xinzweb/gpdb,zeroae/postgres-xl,oberstet/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,xinzweb/gpdb,adam8157/gpdb,yazun/postgres-xl,postmind-net/postgres-xl,snaga/postgres-xl,ovr/postgres-xl,lisakowen/gpdb,zeroae/postgres-xl,xinzweb/gpdb,tpostgres-projects/tPostgres,Postgres-XL/Postgres-XL,pavanvd/postgres-xl,adam8157/gpdb,postmind-net/postgres-xl,lisakowen/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,tpostgres-projects/tPostgres,ashwinstar/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb,pavanvd/postgres-xl,snaga/postgres-xl,50wu/gpdb,jmcatamney/gpdb,tpostgres-projects/tPostgres,techdragon/Postgres-XL