pgsql: Remove uses of "slave" in replication contexts

Remove uses of "slave" in replication contexts

This affects mostly code comments, some documentation, and tests.
Official APIs already used "standby".

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a1ef920e27ba6ab3602aaf6d6751d8628fac1af8

Modified Files
--------------
contrib/dblink/expected/dblink.out | 6 +-
contrib/dblink/sql/dblink.sql | 6 +-
doc/src/sgml/high-availability.sgml | 2 +-
doc/src/sgml/ref/pgupgrade.sgml | 4 +-
src/backend/access/transam/commit_ts.c | 2 +-
src/backend/access/transam/multixact.c | 2 +-
src/backend/access/transam/varsup.c | 4 +-
src/backend/catalog/namespace.c | 4 +-
src/backend/commands/variable.c | 4 +-
src/backend/executor/execMain.c | 4 +-
src/backend/replication/walsender.c | 2 +-
src/backend/storage/ipc/procarray.c | 2 +-
src/backend/tcop/utility.c | 2 +-
src/bin/pg_dump/pg_dump.c | 2 +-
src/bin/pg_rewind/RewindTest.pm | 2 +-
src/test/modules/commit_ts/t/002_standby.pl | 4 +-
src/test/modules/commit_ts/t/003_standby_2.pl | 2 +-
src/test/recovery/t/002_archiving.pl | 2 +-
src/test/recovery/t/005_replay_delay.pl | 2 +-
src/test/recovery/t/009_twophase.pl | 86 +++++++++++++--------------
src/test/recovery/t/012_subtransactions.pl | 62 +++++++++----------
21 files changed, 103 insertions(+), 103 deletions(-)