pgsql: Convert buffer manager to use the new shmem allocation functions

Convert buffer manager to use the new shmem allocation functions

This rectifies the initialization functions a little, making the
"buffer strategy" stuff in freelist.c and buffer mapping hash table in
buf_init.c top-level "subsystems" of their own, registered directly in
subsystemlist.h. Previously they were called indirectly from
BufferManagerShmemInit() and BufferManagerShmemSize()

Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a4b6139dcceb47986577aef36e73f15187ceb727

Modified Files
--------------
src/backend/storage/buffer/buf_init.c | 149 ++++++++++++++-------------------
src/backend/storage/buffer/buf_table.c | 54 ++++++------
src/backend/storage/buffer/freelist.c | 93 +++++++-------------
src/backend/storage/ipc/ipci.c | 3 -
src/include/storage/buf_internals.h | 5 --
src/include/storage/bufmgr.h | 4 -
src/include/storage/subsystemlist.h | 3 +
7 files changed, 124 insertions(+), 187 deletions(-)