Fix overflow with pgstats DSA reference count

When pgstats is initialized for a backend, it uses dsa_attach_in_place()
without a "segment" provided.  Hence, no callback is registered to
automatically release the DSA attached once a backend exits.  Not doing
any cleanup causes the reference count of the pgstats DSA to
continuously increment, at some point overflowing it (the more the
number of connections, the faster it is to reach this state).  Once the
reference count overflows and then gets back to 0, new backends are not
able to attach to the pgstats DSA, failing startup.

This issue is resolved by adding in the pgstats shutdown hook a call to
dsa_release_in_place(), ensuring that the DSA attached at backend
startup is correctly released, keeping the reference count at bay.

The author of this patch has been able to see this issue on a server
with a long uptime and a high connection turnover.

Issue introduced by 5891c7a8ed, so backpatch down to 15.

Author: Anthonin Bonnefoy
Discussion: https://postgr.es/m/CAO6_XqqJbJBL=M7Ym13TcB4Xnq58vRa2jcC+gwEPBgbAda6B1Q@mail.gmail.com
Backpatch-through: 15

src/backend/utils/activity/pgstat_shmem.c

@@ -246,6 +246,14 @@ pgstat_detach_shmem(void)
 	pgStatLocal.shared_hash = NULL;
 
 	dsa_detach(pgStatLocal.dsa);
+
+	/*
+	 * dsa_detach() does not decrement the DSA reference count as no segment
+	 * was provided to dsa_attach_in_place(), causing no cleanup callbacks to
+	 * be registered.  Hence, release it manually now.
+	 */
+	dsa_release_in_place(pgStatLocal.shmem->raw_dsa_area);
+
 	pgStatLocal.dsa = NULL;
 }