[MEDIUM] Collect & provide separate statistics for sockets, v2

This patch allows to collect & provide separate statistics for each socket.
It can be very useful if you would like to distinguish between traffic
generate by local and remote users or between different types of remote
clients (peerings, domestic, foreign).

Currently no "Session rate" is supported, but adding it should be possible
if we found it useful.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 432111b..1c73d24 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -121,6 +121,7 @@
 #define PR_O2_CLFLOG	0x00000400      /* log into clf format */
 #define PR_O2_LOGHCHKS	0x00000800	/* log health checks */
 #define PR_O2_INDEPSTR	0x00001000	/* independant streams, don't update rex on write */
+#define PR_O2_SOCKSTAT	0x00002000	/* collect & provide separate statistics for sockets */
 
 struct error_snapshot {
 	struct timeval when;		/* date of this event, (tv_sec == 0) means "never" */