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>