Convert AIO to use the new shmem allocation functions

Convert AIO to use the new shmem allocation functions

This replaces the "shmem_size" and "shmem_init" callbacks in the IO
methods table with the same ShmemCallback struct that we now use in
other subsystems

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