Minor memory shavings on ESP8266; ~2.5K less

 - Only allow one TCP segment in flight. Each segment is 1.5K and it
   adds up. This may increase latency, but memory is precious on ESP.
   At teh same time, optimize use of tcp_output: do not send a new segment
   on every write, only call tcp_output at the end of connection
   processing during poll.

 - Build LWIP with DNS disabled. We have our won resolver, and LWIP's
   allocates large static buffers (1K).

 - mbedTLS: Deallocate peer's certificate at the end of SSL handshake,
   reduces idle SSL connection footprint by ~1.5K.

 Some tweaks to the heap log viewer (which made all the above possible).

PUBLISHED_FROM=6e84bba64eacb2d737561e09313918104921ea80

mongoose.c

@@ -4270,6 +4270,7 @@ const char *mg_set_ssl(struct mg_connection *nc, const char *cert,
 
 #include <mbedtls/debug.h>
 #include <mbedtls/ecp.h>
+#include <mbedtls/platform.h>
 #include <mbedtls/ssl.h>
 #include <mbedtls/x509_crt.h>
 
@@ -4434,6 +4435,15 @@ enum mg_ssl_if_result mg_ssl_if_handshake(struct mg_connection *nc) {
   }
   err = mbedtls_ssl_handshake(ctx->ssl);
   if (err != 0) return mg_ssl_if_mbed_err(nc, err);
+#ifdef MG_SSL_IF_MBEDTLS_FREE_PEER_CERT
+  /*
+   * Free the peer certificate, we don't need it after handshake.
+   * Note that this effectively disables renegotiation.
+   */
+  mbedtls_x509_crt_free(ctx->ssl->session->peer_cert);
+  mbedtls_free(ctx->ssl->session->peer_cert);
+  ctx->ssl->session->peer_cert = NULL;
+#endif
   return MG_SSL_OK;
 }
 
@@ -13824,8 +13834,21 @@ int mg_lwip_tcp_write(struct mg_connection *nc, const void *data,
     tcp_output(tpcb);
     return 0;
   }
+/*
+ * On ESP8266 we only allow one TCP segment in flight at any given time.
+ * This may increase latency and reduce efficiency of tcp windowing,
+ * but memory is scarce and precious on that platform so we do this to
+ * reduce footprint.
+ */
+#if CS_PLATFORM == CS_P_ESP8266
+  if (tpcb->unacked != NULL) {
+    return 0;
+  }
+  if (tpcb->unsent != NULL) {
+    len = MIN(len, (TCP_MSS - tpcb->unsent->len));
+  }
+#endif
   err_t err = tcp_write(tpcb, data, len, TCP_WRITE_FLAG_COPY);
-  tcp_output(tpcb);
   DBG(("%p tcp_write %u = %d", tpcb, len, err));
   if (err != ERR_OK) {
     /*
@@ -14119,7 +14142,6 @@ time_t mg_lwip_if_poll(struct mg_iface *iface, int timeout_ms) {
   mg_ev_mgr_lwip_process_signals(mgr);
   for (nc = mgr->active_connections; nc != NULL; nc = tmp) {
     struct mg_lwip_conn_state *cs = (struct mg_lwip_conn_state *) nc->sock;
-    (void) cs;
     tmp = nc->next;
     n++;
     if (nc->flags & MG_F_CLOSE_IMMEDIATELY) {
@@ -14154,12 +14176,17 @@ time_t mg_lwip_if_poll(struct mg_iface *iface, int timeout_ms) {
         }
       }
     } else
-#endif /* KR_VERSION */
+#endif /* MG_ENABLE_SSL */
     {
       if (!(nc->flags & (MG_F_CONNECTING | MG_F_UDP))) {
         if (nc->send_mbuf.len > 0) mg_lwip_send_more(nc);
       }
     }
+    if (nc->sock != INVALID_SOCKET &&
+        !(nc->flags & (MG_F_UDP | MG_F_LISTENING)) && cs->pcb.tcp != NULL &&
+        cs->pcb.tcp->unsent != NULL) {
+      tcp_output(cs->pcb.tcp);
+    }
     if (nc->ev_timer_time > 0) {
       if (num_timers == 0 || nc->ev_timer_time < min_timer) {
         min_timer = nc->ev_timer_time;