Move ProcStructLock to the ProcGlobal struct

Move ProcStructLock to the ProcGlobal struct

It protects the freeProcs and some other fields in ProcGlobal, so
let's move it there. It's good for cache locality to have it next to
the thing it protects, and just makes more sense anyway. I believe it
was allocated as a separate shared memory area just for historical
reasons.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/b78719db-0c54-409f-b185-b0d59261143f@iki.fi