Make timestamp search in org-entry-properties more efficient.

* lisp/org.el: (org-entry-properties) Stop scanning for timestamps if
a specific timestamp property (e.g., DEADLINE, SCHEDULED, etc.) is
requested and a match is found. Also, if a specific timestamp property
is requested, do not push non-relevant timestamps onto property list.

This change only effects org-entry-properties when a specific
timestamp is requested with the special flag, as in:

(org-entry-properties nil 'special "SCHEDULED")

Previously, even if only the SCHEDULED timestamp was requested,
org-entry-properties would parse all the timestamps in an entry. This
extra parsing could slow down the construction of agenda views,
especially with entries that contained a large number of log
items (CLOCK, state changes, etc.). The function org-entry-get,
however, is only interested in the first occurrence of the item. When
looking for a specific type of timestamp, org-entry-properties now
stops searching for timestamps after the match is found, unless the
property is "CLOCK".

Here are the relevant ELP results:

Before:

org-entry-get	     296         0.4724579999  0.0015961418
org-entry-properties 31          0.3438769999  0.0110928064

After:

org-entry-get        296         0.1447729999  0.0004890979
org-entry-properties 31          0.015765      0.0005085483

lisp/org.el

@@ -1,4 +1,4 @@
-;;; org.el --- Outline-based notes management and organizer
+';;; org.el --- Outline-based notes management and organizer
 ;; Carstens outline-mode for keeping track of everything.
 ;; Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010
 ;;   Free Software Foundation, Inc.
@@ -13424,32 +13424,38 @@ things up because then unnecessary parsing is avoided."
 		      (member specific
 			      '("SCHEDULED" "DEADLINE" "CLOCK" "CLOSED"
 				"TIMESTAMP" "TIMESTAMP_IA")))
-	      (while (re-search-forward org-maybe-keyword-time-regexp end t)
-		(setq key (if (match-end 1)
-			      (substring (org-match-string-no-properties 1)
-					 0 -1))
-		      string (if (equal key clockstr)
-				 (org-no-properties
-				  (org-trim
-				   (buffer-substring
-				    (match-beginning 3) (goto-char
-							 (point-at-eol)))))
-			       (substring (org-match-string-no-properties 3)
-					  1 -1)))
-		;; Get the correct property name from the key.  This is
-		;; necessary if the user has configured time keywords.
-		(setq key1 (concat key ":"))
-		(cond
-		 ((not key)
-		  (setq key
-			(if (= (char-after (match-beginning 3)) ?\[)
-			    "TIMESTAMP_IA" "TIMESTAMP")))
-		 ((equal key1 org-scheduled-string) (setq key "SCHEDULED"))
-		 ((equal key1 org-deadline-string)  (setq key "DEADLINE"))
-		 ((equal key1 org-closed-string)    (setq key "CLOSED"))
-		 ((equal key1 org-clock-string)     (setq key "CLOCK")))
-		(when (or (equal key "CLOCK") (not (assoc key props)))
-		  (push (cons key string) props))))
+	      (catch 'match
+		(while (re-search-forward org-maybe-keyword-time-regexp end t)
+		  (setq key (if (match-end 1)
+				(substring (org-match-string-no-properties 1)
+					   0 -1))
+			string (if (equal key clockstr)
+				   (org-no-properties
+				    (org-trim
+				     (buffer-substring
+				      (match-beginning 3) (goto-char
+							   (point-at-eol)))))
+				 (substring (org-match-string-no-properties 3)
+					    1 -1)))
+		  ;; Get the correct property name from the key.  This is
+		  ;; necessary if the user has configured time keywords.
+		  (setq key1 (concat key ":"))
+		  (cond
+		   ((not key)
+		    (setq key
+			  (if (= (char-after (match-beginning 3)) ?\[)
+			      "TIMESTAMP_IA" "TIMESTAMP")))
+		   ((equal key1 org-scheduled-string) (setq key "SCHEDULED"))
+		   ((equal key1 org-deadline-string)  (setq key "DEADLINE"))
+		   ((equal key1 org-closed-string)    (setq key "CLOSED"))
+		   ((equal key1 org-clock-string)     (setq key "CLOCK")))
+		  (if (and specific (equal key specific) (not (equal key "CLOCK")))
+		      (progn 
+			(push (cons key string) props)
+			;; no need to search further if match is found
+			(throw 'match t))
+		    (when (or (equal key "CLOCK") (not (assoc key props)))
+		      (push (cons key string) props))))))
 	    )
 
 	  (when (memq which '(all standard))