Remove concept of multiple timer managers

- All timers are now handled by a single global timer manager, which simplifies how they handled by the IOSource manager.
- This change flows down a number of changes to other parts of the code. The timer manager tag field is removed, which means that matching connections to a timer manager is also removed. This removes the ability to tag a connection as internal or external, since that's how the connections where differentiated. This in turn removes the `current_conns_extern` field from the `ConnStats` record type in the script layer.

src/main.cc

@@ -550,7 +550,7 @@ int main(int argc, char** argv)
 	createCurrentDoc("1.0");		// Set a global XML document
 #endif
 
-	timer_mgr = new PQ_TimerMgr("<GLOBAL>");
+	timer_mgr = new PQ_TimerMgr();
 
 	auto zeekygen_cfg = options.zeekygen_config_file.value_or("");
 	zeekygen_mgr = new zeekygen::Manager(zeekygen_cfg, bro_argv[0]);