[MAJOR] convert all expiration timers from timeval to ticks

This is the first attempt at moving all internal parts from
using struct timeval to integer ticks. Those provides simpler
and faster code due to simplified operations, and this change
also saved about 64 bytes per session.

A new header file has been added : include/common/ticks.h.

It is possible that some functions should finally not be inlined
because they're used quite a lot (eg: tick_first, tick_add_ifset
and tick_is_expired). More measurements are required in order to
decide whether this is interesting or not.

Some function and variable names are still subject to change for
a better overall logics.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 5b0914f..cc647a6 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -2,7 +2,7 @@
   include/types/proto_http.h
   This file contains HTTP protocol definitions.
 
-  Copyright (C) 2000-2006 Willy Tarreau - w@1wt.eu
+  Copyright (C) 2000-2008 Willy Tarreau - w@1wt.eu
   
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
@@ -240,7 +240,7 @@
 	char *srv_cookie;		/* cookie presented by the server, in capture mode */
 	int status;			/* HTTP status from the server, negative if from proxy */
 	unsigned int flags;             /* transaction flags */
-	struct timeval exp;             /* expiration date for the transaction (generally a request) */
+	int exp;                        /* expiration date for the transaction (generally a request), int ticks */
 };
 
 /* This structure is used by http_find_header() to return values of headers.