pgsql: Optimize shared memory usage for WaitLSNProcInfo

Optimize shared memory usage for WaitLSNProcInfo

We need separate pairing heaps for different WaitLSNType's, because there
might be waiters for different LSN's at the same time.  However, one process
can wait only for one type of LSN at a time.  So, no need for inHeap
and heapNode fields to be arrays.

Discussion: https://postgr.es/m/CAPpHfdsBR-7sDtXFJ1qpJtKiohfGoj%3DvqzKVjWxtWsWidx7G_A%40mail.gmail.com
Author: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/75e82b2f5a6f5de6b42dbd9ea73be5ff36a931b1

Modified Files
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src/backend/access/transam/xlogwait.c | 42 ++++++++++++++++-------------------
src/include/access/xlogwait.h         | 14 ++++++++----
2 files changed, 29 insertions(+), 27 deletions(-)