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