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