Rename PqMsg_Progress to PqMsg_ParallelWorkerProgress

Sami Imseih <samimseih.pg@gmail.com> writes:

While looking at this, I realized that "PqMsg_Progress" is the name of the
'P' message that was added for parallel index progress reporting.

I missed [1] at the time, but if there is still a chance to rename it to
"PqMsg_ParallelWorkerProgress", that seems better,
"PqMsg_Progress" does not really describe what it is, whereas
"PqMsg_ParallelWorkerProgress" makes it explicit that this is a
parallel-worker-to-leader message. It also matches other places in the code
that deal with parallel workers, such as "IsParallelWorker", so it is easier
to find when looking through parallel-worker code.

Yeah, "PqMsg_Progress" seems unduly generic. I don't think we should
rename it in the back branches, but doing so in master looks like
it'd be a small-footprint change.

regards, tom lane