pgsql: Revert lowering of lock level for ATTACH PARTITION

Revert lowering of lock level for ATTACH PARTITION

I lowered the lock level for partitions being scanned from
AccessExclusive to ShareLock in the course of 72cf7f310c07, but that was
bogus, as pointed out by Robert Haas.  Revert that bit.  Doing this is
possible, but requires more work.

Discussion: https://postgr.es/m/CA+TgmobV7Nfmqv+TZXcdSsb9Bjc-OL-Anv6BNmCbfJVZLYPE4Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b8ca984b2c739e096c08f260f477792495f4dfe4

Modified Files
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src/backend/commands/tablecmds.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)