pgsql: Make CLUSTER lock the old table's toast table before copying dat

Make CLUSTER lock the old table's toast table before copying data.

We must lock out autovacuuming of the old toast table before computing the
OldestXmin horizon we will use.  Otherwise, autovacuum could start on the
toast table later, compute a later OldestXmin horizon, and remove as DEAD
toast tuples that we still need (because we think their parent tuples are
only RECENTLY_DEAD).  Per further thought about bug #5998.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fd384cfc34b0b2e393c5d9c92acb0600f5a3e5b8

Modified Files
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src/backend/commands/cluster.c |   16 ++++++++++++++++
1 files changed, 16 insertions(+), 0 deletions(-)