Add a changelog entry for PR #2590 and further optimize log_message()

CI_Log::write_log() already checks the log threshold, so there's no point in doing it in log_message() as well.
diff --git a/system/core/Common.php b/system/core/Common.php
index 98a3fcb..21e1df9 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -439,17 +439,7 @@
 	 */
 	function log_message($level, $message, $php_error = FALSE)
 	{
-		static $_log, $_log_threshold;
-
-		if ($_log_threshold === NULL)
-		{
-			$_log_threshold = config_item('log_threshold');
-		}
-
-		if ($_log_threshold === 0)
-		{
-			return;
-		}
+		static $_log;
 
 		if ($_log === NULL)
 		{