pgsql: Make struct ParallelSlot private within pg_dump/parallel.c.

Make struct ParallelSlot private within pg_dump/parallel.c.

The only field of this struct that other files have any need to touch
is the pointer to the TocEntry a worker is working on.  (Well,
pg_backup_archiver.c is actually looking at workerStatus too, but that
can be finessed by specifying that the TocEntry pointer is NULL for a
non-busy worker.)

Hence, move out the TocEntry pointers to a separate array within
struct ParallelState, and then we can make struct ParallelSlot private.

I noted the possibility of this previously, but hadn't got round to
actually doing it.

Discussion: <1188.1464544443@sss.pgh.pa.us>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0109ab27609c0d58c1eddc6b799077d0968083de

Modified Files
--------------
src/bin/pg_dump/parallel.c           | 62 +++++++++++++++++++++++++++++++-----
src/bin/pg_dump/parallel.h           | 45 +++-----------------------
src/bin/pg_dump/pg_backup_archiver.c | 12 +++----
3 files changed, 65 insertions(+), 54 deletions(-)