pgsql: pg_dump: Fix query for gathering attribute stats on older versio

pg_dump: Fix query for gathering attribute stats on older versions.

Commit 9c02e3a986 taught pg_dump to retrieve attribute statistics
for 64 relations at a time. pg_dump supports dumping from v9.2 and
newer versions, but our query for retrieving statistics for
multiple relations uses WITH ORDINALITY and multi-argument
UNNEST(), both of which were introduced in v9.4. To fix, we resort
to gathering statistics for a single relation at a time on versions
older than v9.4.

Per buildfarm member crake.

Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Discussion: https://postgr.es/m/Z_BcWVMvlUIJ_iuZ%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f0d0083f52f994989b1bb6723e7bd0fad9f586cd

Modified Files
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src/bin/pg_dump/pg_dump.c | 38 ++++++++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 8 deletions(-)