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.huinker@gmail.com>
Discussion: https://postgr.es/m/Z_BcWVMvlUIJ_iuZ%40nathan

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