pgsql: Move ShmemIndexLock into ShmemAllocator

Move ShmemIndexLock into ShmemAllocator

This makes shmem.c independent of the main LWLock array. That makes it
possible to stop passing MainLWLockArray through BackendParameters in
the next commit.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/47aaf57e-1b7b-4e12-bda2-0316081ff50e@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/06d859aaf4e988ddef8adc55efb21324c5e30092

Modified Files
--------------
src/backend/storage/ipc/shmem.c                 | 15 ++++++++++-----
src/backend/utils/activity/wait_event_names.txt |  2 +-
src/include/storage/lwlocklist.h                |  3 ++-
3 files changed, 13 insertions(+), 7 deletions(-)