Request For Feature: pg_dump

In --format=directory mode, remove .dat files with zero data records, and
mark that table's toc.dat entry that it's an empty table.

Justification: *lots* of empty tables means *lots* of teeny-tiny files in
the DB's dump directory. That unnecessarily bloats the fs, and makes "du
-c" really really slow.

But why are there sooo many empty tables? You shouldn't have so many empty
tables!

Yeah, well, software (especially 3rd-party software that must be generic to
satisfy the varying needs of a large and varied customer base) can't always
be perfectly tuned to the precise and immediate needs of a particular
site. Partitioning makes that much much worse.

We've survived this long without it, but pgbackrest has a similar feature
(though implemented differently from how pg_dump would do it), and it's
*really* handy.

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