MEDIUM: sessions: Keep track of which connections are idle.

Instead of keeping track of the number of connections we're responsible for,
keep track of the number of connections we're responsible for that we are
currently considering idling (ie that we are not using, they may be in use
by other sessions), that way we can actually reuse connections when we have
more connections than the max configured.
diff --git a/include/types/connection.h b/include/types/connection.h
index f1fd7a0..b273ece 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -196,6 +196,8 @@
 	/* This flag is used to know that a PROXY protocol header was sent by the client */
 	CO_FL_RCVD_PROXY    = 0x20000000,
 
+	/* The connection is unused by its owner */
+	CO_FL_SESS_IDLE     = 0x40000000,
 	/* unused : 0x40000000 */
 
 	/* This last flag indicates that the transport layer is used (for instance