EventMgr: Remove queue_flare, use GetNextTimeout() instead

It can be visible overhead to call write() on the underlying pipe of the
EventMgr's flare whenever the first event is enqueued during an IO loop
iteration. Particularly in scenarios where there's about 1 event per packet
for long lived connections and script-side event processing is fast.

Given the event manager is drained anyhow at the end of the main loop, this
shouldn't be needed. In fact, the EventMgr.Process() method is basically
a stub. The one reason it is needed is when more events are enqueued during
a drain. That, however, can be dealt with by implementing GetNextTimeout()
to return 0.0 when there's more events queued. This way the main-loop's poll
timeout is 0.0 and it'll continue immediately.

This also allows to removes some extra code and drop the recently introduced
InitPostFork() addition: Without a pipe, there's no need to recreate it.

src/zeek-setup.cc

@@ -506,8 +506,6 @@ SetupResult setup(int argc, char** argv, Options* zopts) {
         // If we get here, we're a supervised node that just returned
         // from CreateStem() after being forked from the stem.
         Supervisor::ThisNode()->Init(&options);
-
-        event_mgr.InitPostFork();
     }
 
     script_coverage_mgr.ReadStats();