pgsql: Rename PqMsg_Progress to PqParallelMsg_Progress.

Rename PqMsg_Progress to PqParallelMsg_Progress.

The previous name was considered to be unduly generic. This commit
addresses that by adding a new "PqParallelMsg" prefix for protocol
characters sent by parallel workers to leader processes. The new
prefix can be reused for new parallel worker message types added in
the future.

Author: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Antonin Houska <ah(at)cybertec(dot)at>
Discussion: https://postgr.es/m/CAN12%2BYLqbiA1v3FPD94yLbXgdyoWxVEY6%3DDtdpTPo9YCRuCpfg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8367b63e2cabb69ca0f9a5a6e6f04a1869b525a3

Modified Files
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src/backend/access/transam/parallel.c | 2 +-
src/backend/utils/activity/backend_progress.c | 8 ++++----
src/include/libpq/protocol.h | 4 ++--
3 files changed, 7 insertions(+), 7 deletions(-)