define PG_REPLSLOT_DIR

Hi hackers,

while working on a replication slot tool (idea is to put it in contrib, not
shared yet), I realized that "pg_replslot" is being used > 25 times in
.c files.

I think it would make sense to replace those occurrences with a $SUBJECT, attached
a patch doing so.

There is 2 places where it is not done:

src/bin/initdb/initdb.c
src/bin/pg_rewind/filemap.c

for consistency with the existing PG_STAT_TMP_DIR define.

Out of curiosity, checking the sizes of affected files (O2, no debug):

with patch:

text data bss dec hex filename
20315 224 17 20556 504c src/backend/backup/basebackup.o
30304 0 8 30312 7668 src/backend/replication/logical/reorderbuffer.o
23812 36 40 23888 5d50 src/backend/replication/slot.o
6367 0 0 6367 18df src/backend/utils/adt/genfile.o
40997 2574 2528 46099 b413 src/bin/initdb/initdb.o
6963 224 8 7195 1c1b src/bin/pg_rewind/filemap.o

without patch:

text data bss dec hex filename
20315 224 17 20556 504c src/backend/backup/basebackup.o
30286 0 8 30294 7656 src/backend/replication/logical/reorderbuffer.o
23766 36 40 23842 5d22 src/backend/replication/slot.o
6363 0 0 6363 18db src/backend/utils/adt/genfile.o
40997 2574 2528 46099 b413 src/bin/initdb/initdb.o
6963 224 8 7195 1c1b src/bin/pg_rewind/filemap.o

Also, I think we could do the same for:

pg_notify
pg_serial
pg_subtrans
pg_wal
pg_multixact
pg_tblspc
pg_logical

And I volunteer to do so if you think that makes sense.

Looking forward to your feedback,

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com