MEDIUM: connection: replace idle conn lists by eb trees

The server idle/safe/available connection lists are replaced with ebmb-
trees. This is used to store backend connections, with the new field
connection hash as the key. The hash is a 8-bytes size field, used to
reflect specific connection parameters.

This is a preliminary work to be able to reuse connection with SNI,
explicit src/dst address or PROXY protocol.
diff --git a/src/haproxy.c b/src/haproxy.c
index cdbc3dc..7a8e063 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2658,9 +2658,9 @@
 			free(s->hostname);
 			free(s->hostname_dn);
 			free((char*)s->conf.file);
-			free(s->idle_conns);
-			free(s->safe_conns);
-			free(s->available_conns);
+			free(s->idle_conns_tree);
+			free(s->safe_conns_tree);
+			free(s->available_conns_tree);
 			free(s->curr_idle_thr);
 			free(s->resolvers_id);
 			free(s->addr_node.key);