Convert lwlock.c to use the new shmem allocation functions

Convert lwlock.c to use the new shmem allocation functions

It seems like a good candidate to convert first because it needs to
initialized before any other subsystem, but other than that it's
nothing special.

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