lwip2 updates: no more git sub-sub-module deps, faster checksum, backlog limitation and other fixes (#6887)

* upstream lwIP is now downloaded by a makefile, not subsubmoduled

* lwip2: upstream lwIP not sub-sub-modules anymore
lwip2: Allow IPv4 and IPv6 DNS and SNTP server configured via DHCP to co-exist (patch against upstream)

* lwip2: enable tcp-listen-with-backlog feature

* lwip2 submodule update:
- enable more efficient chksum algorithm thanks to Richard Allen
- enable tcp listener with backlog

* more comments, fix backlog management, fix API
* move default value definition in .cpp
because one must not believe it can be redefined before including WiFiServer.h

* improved backlog handling, it is no more a breaking change

libraries/ESP8266WiFi/src/WiFiServer.h

@@ -31,6 +31,37 @@ extern "C" {
 #include "Server.h"
 #include "IPAddress.h"
 
+// lwIP-v2 backlog facility allows to keep memory safe by limiting the
+// maximum number of incoming *pending clients*.  Default number of possibly
+// simultaneously pending clients is defined in WiFiServer.cpp
+// (MAX_PENDING_CLIENTS_PER_PORT=5).  User can overide it at runtime from
+// sketch:
+//      WiFiServer::begin(port, max-simultaneous-pending-clients);
+//
+// An "incoming pending" client is a new incoming TCP connection trying to
+// reach the TCP server.  It is "pending" until lwIP acknowledges it and
+// "accepted / no more pending" when user calls WiFiServer::available().
+//
+// Before the backlog feature or with lwIP-v1.4, there was no pending
+// connections: They were immediately accepted and filling RAM.
+//
+// Several pending clients can appear during the time when one client is
+// served by a long not-async service like ESP8266WebServer.  During that
+// time WiFiServer::available() cannot be called.
+//
+// Note: This *does not limit* the number of *simultaneously accepted
+//       clients*.  Such limit management is left to the user.
+//
+// Thus, when the maximum number of pending connections is reached, new
+// connections are delayed.
+// By "delayed", it is meant that WiFiServer(lwIP) will not answer to the
+// SYN packet until there is room for a new one: The TCP server on that port
+// will be mute.  The TCP client will regularly try to connect until success
+// or a timeout occurs (72s on windows).
+//
+// When user calls WiFiServer::available(), the tcp server stops muting and
+// answers to newcomers (until the "backlog" pending list is full again).
+
 class ClientContext;
 class WiFiClient;
 
@@ -39,7 +70,7 @@ class WiFiServer : public Server {
 protected:
   uint16_t _port;
   IPAddress _addr;
-  tcp_pcb* _pcb;
+  tcp_pcb* _listen_pcb;
 
   ClientContext* _unclaimed;
   ClientContext* _discarded;
@@ -53,6 +84,7 @@ public:
   bool hasClient();
   void begin();
   void begin(uint16_t port);
+  void begin(uint16_t port, uint8_t backlog);
   void setNoDelay(bool nodelay);
   bool getNoDelay();
   virtual size_t write(uint8_t);