[MINOR] call session->do_log() for logging

In order to avoid having to call per-protocol logging function directly
from session.c, it's better to assign the logging function when the session
is created. This also eliminates a test when the function is needed, and
opens the way to more complete logging functions.
diff --git a/src/client.c b/src/client.c
index 78387a1..8e8dc53 100644
--- a/src/client.c
+++ b/src/client.c
@@ -206,6 +206,11 @@
 		else
 			s->logs.logwait = p->to_log;
 
+		if (s->logs.logwait & LW_REQ)
+			s->do_log = http_sess_log;
+		else
+			s->do_log = tcp_sess_log;
+
 		s->logs.accept_date = date; /* user-visible date for logging */
 		s->logs.tv_accept = now;  /* corrected date for internal use */
 		tv_zero(&s->logs.tv_request);
@@ -276,7 +281,7 @@
 				/* we have the client ip */
 				if (s->logs.logwait & LW_CLIP)
 					if (!(s->logs.logwait &= ~LW_CLIP))
-						tcp_sess_log(s);
+						s->do_log(s);
 			}
 			else if (s->cli_addr.ss_family == AF_INET) {
 				char pn[INET_ADDRSTRLEN], sn[INET_ADDRSTRLEN];