Revert "graceful shutdown" changes for Windows, in back branches only.

This reverts commits 6051857fc and ed52c3707, but only in the back
branches.  Further testing has shown that while those changes do fix
some things, they also break others; in particular, it looks like
walreceivers fail to detect walsender-initiated connection close
reliably if the walsender shuts down this way.  We'll keep trying to
improve matters in HEAD, but it now seems unwise to push these changes
into stable releases.

Discussion: https://postgr.es/m/CA+hUKG+OeoETZQ=Qw5Ub5h3tmwQhBmDA=nuNO3KG=zWfUypFAw@mail.gmail.com

src/backend/libpq/pqcomm.c

@@ -277,30 +277,15 @@ socket_close(int code, Datum arg)
 		secure_close(MyProcPort);
 
 		/*
-		 * On most platforms, we leave the socket open until the process dies.
-		 * This allows clients to perform a "synchronous close" if they care
-		 * --- wait till the transport layer reports connection closure, and
-		 * you can be sure the backend has exited.  Saves a kernel call, too.
+		 * Formerly we did an explicit close() here, but it seems better to
+		 * leave the socket open until the process dies.  This allows clients
+		 * to perform a "synchronous close" if they care --- wait till the
+		 * transport layer reports connection closure, and you can be sure the
+		 * backend has exited.
 		 *
-		 * However, that does not work on Windows: if the kernel closes the
-		 * socket it will invoke an "abortive shutdown" that discards any data
-		 * not yet sent to the client.  (This is a flat-out violation of the
-		 * TCP RFCs, but count on Microsoft not to care about that.)  To get
-		 * the spec-compliant "graceful shutdown" behavior, we must invoke
-		 * closesocket() explicitly.  When using OpenSSL, it seems that clean
-		 * shutdown also requires an explicit shutdown() call.
-		 *
-		 * This code runs late enough during process shutdown that we should
-		 * have finished all externally-visible shutdown activities, so that
-		 * in principle it's good enough to act as a synchronous close on
-		 * Windows too.  But it's a lot more fragile than the other way.
+		 * We do set sock to PGINVALID_SOCKET to prevent any further I/O,
+		 * though.
 		 */
-#ifdef WIN32
-		shutdown(MyProcPort->sock, SD_SEND);
-		closesocket(MyProcPort->sock);
-#endif
-
-		/* In any case, set sock to PGINVALID_SOCKET to prevent further I/O */
 		MyProcPort->sock = PGINVALID_SOCKET;
 	}
 }