Made mercenary db dynamic (#2883)

Removed hard limit of the mercenary database
Cleaned up the hard coded duplicated logic
Turned hard coded mercenary ids into constants

src/map/script.cpp

@@ -18980,7 +18980,7 @@ BUILDIN_FUNC(mercenary_create)
 
 	class_ = script_getnum(st,2);
 
-	if( !mercenary_class(class_) )
+	if( !mercenary_db(class_) )
 		return SCRIPT_CMD_SUCCESS;
 
 	contract_time = script_getnum(st,3);