TimerMgr: Add back max_timer_expires=0 special case

Commit 58fae22708 removed the max_expire==0
handling from DoAdvance() due to not being obvious what use it is. Jan
later reported that it broke the `redef max_timer_expires=0` (#2514).

This commit adds back the special case re-introducing the `max_timer_expires=0` ,
trying to make it fairly explicit that it exists.

This is an adaption of #2516 not adding a new option and trying a bit
to avoid global variable accesses down in DoAdvance(), though that
just moved to InitPostScript().

Fixes #2514.

scripts/base/init-bare.zeek

@@ -2116,7 +2116,7 @@ global discarder_check_icmp: function(p: pkt_hdr): bool;
 ## Zeek's watchdog interval.
 const watchdog_interval = 10 sec &redef;
 
-## The maximum number of timers to expire after processing each new
+## The maximum number of expired timers to process after processing each new
 ## packet. The value trades off spreading out the timer expiration load
 ## with possibly having to hold state longer.  A value of 0 means
 ## "process all expired timers with each new packet".

src/Timer.cc

@@ -111,6 +111,8 @@ void TimerMgr::InitPostScript()
 	{
 	if ( iosource_mgr )
 		iosource_mgr->Register(this, true);
+
+	dispatch_all_expired = zeek::detail::max_timer_expires == 0;
 	}
 
 void TimerMgr::Add(Timer* timer)
@@ -142,7 +144,8 @@ void TimerMgr::Expire()
 int TimerMgr::DoAdvance(double new_t, int max_expire)
 	{
 	Timer* timer = Top();
-	for ( num_expired = 0; (num_expired < max_expire) && timer && timer->Time() <= new_t;
+	for ( num_expired = 0;
+	      (num_expired < max_expire || dispatch_all_expired) && timer && timer->Time() <= new_t;
 	      ++num_expired )
 		{
 		last_timestamp = timer->Time();

src/Timer.h

@@ -84,7 +84,8 @@ public:
 	void Add(Timer* timer);
 
 	/**
-	 * Advance the clock to time t, expiring at most max_expire timers.
+	 * Advance the clock to time t, dispatching at most max_expire expired
+	 * timers, or all expired timers if dispatch_all_expired is set.
 	 *
 	 * @param t the new time.
 	 * @param max_expire the maximum number of timers to expire.
@@ -152,6 +153,9 @@ private:
 	double last_advance;
 
 	int num_expired;
+	// Flag to indicate if Advance() should dispatch all expired timers
+	// for the max_timer_expires=0 case.
+	bool dispatch_all_expired = false;
 
 	size_t peak_size = 0;
 	size_t cumulative_num = 0;