Introduce timeout handling framework

Management of timeouts was getting a little cumbersome; what we
originally had was more than enough back when we were only concerned
about deadlocks and query cancel; however, when we added timeouts for
standby processes, the code got considerably messier.  Since there are
plans to add more complex timeouts, this seems a good time to introduce
a central timeout handling module.

External modules register their timeout handlers during process
initialization, and later enable and disable them as they see fit using
a simple API; timeout.c is in charge of keeping track of which timeouts
are in effect at any time, installing a common SIGALRM signal handler,
and calling setitimer() as appropriate to ensure timely firing of
external handlers.

timeout.c additionally supports pluggable modules to add their own
timeouts, though this capability isn't exercised anywhere yet.

Additionally, as of this commit, walsender processes are aware of
timeouts; we had a preexisting bug there that made those ignore SIGALRM,
thus being subject to unhandled deadlocks, particularly during the
authentication phase.  This has already been fixed in back branches in
commit 0bf8eb2a, which see for more details.

Main author: Zoltán Böszörményi
Some review and cleanup by Álvaro Herrera
Extensive reworking by Tom Lane

src/backend/postmaster/startup.c

@@ -27,8 +27,9 @@
 #include "storage/ipc.h"
 #include "storage/latch.h"
 #include "storage/pmsignal.h"
-#include "storage/proc.h"
+#include "storage/standby.h"
 #include "utils/guc.h"
+#include "utils/timeout.h"
 
 
 /*
@@ -185,20 +186,12 @@ StartupProcessMain(void)
 
 	/*
 	 * Properly accept or ignore signals the postmaster might send us.
-	 *
-	 * Note: ideally we'd not enable handle_standby_sig_alarm unless actually
-	 * doing hot standby, but we don't know that yet.  Rely on it to not do
-	 * anything if it shouldn't.
 	 */
 	pqsignal(SIGHUP, StartupProcSigHupHandler); /* reload config file */
 	pqsignal(SIGINT, SIG_IGN);	/* ignore query cancel */
 	pqsignal(SIGTERM, StartupProcShutdownHandler);		/* request shutdown */
 	pqsignal(SIGQUIT, startupproc_quickdie);	/* hard crash time */
-	if (EnableHotStandby)
-		pqsignal(SIGALRM, handle_standby_sig_alarm);	/* ignored unless
-														 * InHotStandby */
-	else
-		pqsignal(SIGALRM, SIG_IGN);
+	InitializeTimeouts();		/* establishes SIGALRM handler */
 	pqsignal(SIGPIPE, SIG_IGN);
 	pqsignal(SIGUSR1, StartupProcSigUsr1Handler);
 	pqsignal(SIGUSR2, StartupProcTriggerHandler);
@@ -212,11 +205,20 @@ StartupProcessMain(void)
 	pqsignal(SIGCONT, SIG_DFL);
 	pqsignal(SIGWINCH, SIG_DFL);
 
+	/*
+	 * Register timeouts needed for standby mode
+	 */
+	RegisterTimeout(STANDBY_DEADLOCK_TIMEOUT, StandbyDeadLockHandler);
+	RegisterTimeout(STANDBY_TIMEOUT, StandbyTimeoutHandler);
+
 	/*
 	 * Unblock signals (they were blocked when the postmaster forked us)
 	 */
 	PG_SETMASK(&UnBlockSig);
 
+	/*
+	 * Do what we came for.
+	 */
 	StartupXLOG();
 
 	/*