pgstat's on-proc-exit hook has to execute after the last transaction commit

pgstat's on-proc-exit hook has to execute after the last transaction commit
or abort within a backend; rearrange InitPostgres processing to make it so.
Revealed by just-added Asserts along with ECPG regression tests (hm, I wonder
why the core regression tests didn't expose it?).  This possibly is another
reason for missing stats updates ...