MINOR: lb: allow redispatch when using consistent hash

Redispatch traditionally only worked for cookie based persistence.

Adding redispatch support for consistent hash based persistence - also
update docs.

Reported by Oskar Stenman on discourse:
https://discourse.haproxy.org/t/balance-uri-consistent-hashing-redispatch-3-not-redispatching/3344

Should be backported to 1.8.

Cc: Lukas Tribus <lukas@ltri.eu>
diff --git a/include/proto/lb_chash.h b/include/proto/lb_chash.h
index a0ebf69..679dff3 100644
--- a/include/proto/lb_chash.h
+++ b/include/proto/lb_chash.h
@@ -28,7 +28,7 @@
 
 void chash_init_server_tree(struct proxy *p);
 struct server *chash_get_next_server(struct proxy *p, struct server *srvtoavoid);
-struct server *chash_get_server_hash(struct proxy *p, unsigned int hash);
+struct server *chash_get_server_hash(struct proxy *p, unsigned int hash, const struct server *avoid);
 
 #endif /* _PROTO_LB_CHASH_H */