Use a long lived WaitEventSet for WaitLatch().

Create LatchWaitSet at backend startup time, and use it to implement
WaitLatch().  This avoids repeated epoll/kqueue setup and teardown
system calls.

Reorder SubPostmasterMain() slightly so that we restore the postmaster
pipe and Windows signal emulation before we reach InitPostmasterChild(),
to make this work in EXEC_BACKEND builds.

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -4896,9 +4896,6 @@ SubPostmasterMain(int argc, char *argv[])
 	IsPostmasterEnvironment = true;
 	whereToSendOutput = DestNone;
 
-	/* Setup as postmaster child */
-	InitPostmasterChild();
-
 	/* Setup essential subsystems (to ensure elog() behaves sanely) */
 	InitializeGUCOptions();
 
@@ -4913,6 +4910,18 @@ SubPostmasterMain(int argc, char *argv[])
 	/* Close the postmaster's sockets (as soon as we know them) */
 	ClosePostmasterPorts(strcmp(argv[1], "--forklog") == 0);
 
+	/*
+	 * Start our win32 signal implementation. This has to be done after we
+	 * read the backend variables, because we need to pick up the signal pipe
+	 * from the parent process.
+	 */
+#ifdef WIN32
+	pgwin32_signal_initialize();
+#endif
+
+	/* Setup as postmaster child */
+	InitPostmasterChild();
+
 	/*
 	 * Set up memory area for GSS information. Mirrors the code in ConnCreate
 	 * for the non-exec case.
@@ -4956,15 +4965,6 @@ SubPostmasterMain(int argc, char *argv[])
 	if (strcmp(argv[1], "--forkavworker") == 0)
 		AutovacuumWorkerIAm();
 
-	/*
-	 * Start our win32 signal implementation. This has to be done after we
-	 * read the backend variables, because we need to pick up the signal pipe
-	 * from the parent process.
-	 */
-#ifdef WIN32
-	pgwin32_signal_initialize();
-#endif
-
 	/* In EXEC_BACKEND case we will not have inherited these settings */
 	pqinitmask();
 	PG_SETMASK(&BlockSig);