pgsql: Use dlists instead of SHM_QUEUE for syncrep queue

Use dlists instead of SHM_QUEUE for syncrep queue

Part of a series to remove SHM_QUEUE. ilist.h style lists are more widely used
and have an easier to use interface.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com> (in an older version)
Discussion: https://postgr.es/m/20221120055930.t6kl3tyivzhlrzu2@awork3.anarazel.de
Discussion: https://postgr.es/m/20200211042229.msv23badgqljrdg2@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/12605414a7d63ccbe36de2b530847bdfc9cf7447

Modified Files
--------------
src/backend/replication/syncrep.c           | 89 ++++++++++++-----------------
src/backend/replication/walsender.c         |  2 +-
src/backend/storage/lmgr/proc.c             |  2 +-
src/include/replication/walsender_private.h |  3 +-
src/include/storage/proc.h                  |  2 +-
5 files changed, 41 insertions(+), 57 deletions(-)