pgsql: Add new parallel message type to progress reporting.

Add new parallel message type to progress reporting.

This commit adds a new type of parallel message 'P' to allow a
parallel worker to poke at a leader to update the progress.

Currently it supports only incremental progress reporting but it's
possible to allow for supporting of other backend progress APIs in the
future.

There are no users of this new message type as of this commit. That
will follow in future commits.

Idea from Andres Freund.

Author: Sami Imseih
Reviewed by: Michael Paquier, Masahiko Sawada
Discussion: https://www.postgresql.org/message-id/flat/5478DFCD-2333-401A-B2F0-0D186AB09228@amazon.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f1889729dd3ab0352dc0ccc2ffcc1b1901f8e39f

Modified Files
--------------
src/backend/access/transam/parallel.c         | 18 +++++++++++++++
src/backend/utils/activity/backend_progress.c | 32 +++++++++++++++++++++++++++
src/include/utils/backend_progress.h          |  1 +
3 files changed, 51 insertions(+)