I lowered the lock level for partitions being scanned from
AccessExclusive to ShareLock in the course of 72cf7f31, 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