Convert pg_stat_statements to use the new shmem allocation functions

Convert pg_stat_statements to use the new shmem allocation functions

As part of this, embed the LWLock it needs in the shared memory struct
itself, so that we don't need to use RequestNamedLWLockTranche()
anymore. LWLockNewTrancheId() + LWLockInitialize() is more convenient
to use in extensions.

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