Fix possible overflow of pg_stat DSA's refcnt

Hi,

During backend initialisation, pgStat DSA is attached using
dsa_attach_in_place with a NULL segment. The NULL segment means that
there's no callback to release the DSA when the process exits.
pgstat_detach_shmem only calls dsa_detach which, as mentioned in the
function's comment, doesn't include releasing and doesn't decrement the
reference count of pgStat DSA.

Thus, every time a backend is created, pgStat DSA's refcnt is incremented
but never decremented when the backend shutdown. It will eventually
overflow and reach 0, triggering the "could not attach to dynamic shared
area" error on all newly created backends. When this state is reached, the
only way to recover is to restart the db to reset the counter.

The issue can be visible by calling dsa_dump in pgstat_detach_shmem and
checking that refcnt's value is continuously increasing as new backends are
created. It is also possible to reach the state where all connections are
refused by editing the refcnt manually with lldb/gdb (The alternative,
creating enough backends to reach 0 exists but can take some time). Setting
it to -10 and then opening 10 connections will eventually generate the
"could not attach" error.

This patch fixes this issue by releasing pgStat DSA with
dsa_release_in_place during pgStat shutdown to correctly decrement the
refcnt.

Regards,
Anthonin