rename ClientContext::wait_until_sent() to wait_until_acked()

While looking at esp8266#6348 and esp8266#6369, and after checking into lwIP sources, it
appears that the tests in ClientContext::wait_until_sent() effectively wait
for all acks on current output buffer. Comments are added.

Author: d-a-v

libraries/ESP8266WiFi/src/WiFiClient.cpp

@@ -304,7 +304,7 @@ bool WiFiClient::flush(unsigned int maxWaitMs)
 
     if (maxWaitMs == 0)
         maxWaitMs = WIFICLIENT_MAX_FLUSH_WAIT_MS;
-    return _client->wait_until_sent(maxWaitMs);
+    return _client->wait_until_acked(maxWaitMs);
 }
 
 bool WiFiClient::stop(unsigned int maxWaitMs)

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -314,7 +314,7 @@ class ClientContext
         _rx_buf_offset = 0;
     }
 
-    bool wait_until_sent(int max_wait_ms = WIFICLIENT_MAX_FLUSH_WAIT_MS)
+    bool wait_until_acked(int max_wait_ms = WIFICLIENT_MAX_FLUSH_WAIT_MS)
     {
         // https://github.com/esp8266/Arduino/pull/3967#pullrequestreview-83451496
         // option 1 done
@@ -351,6 +351,8 @@ class ClientContext
             delay(0); // from sys or os context
 
             if ((state() != ESTABLISHED) || (sndbuf == TCP_SND_BUF)) {
+                // peer has closed or all bytes are sent and acked
+                // ((TCP_SND_BUF-sndbuf) is the amount of un-acked bytes)
                 break;
             }
         }
@@ -484,7 +486,7 @@ class ClientContext
         } while(true);
 
         if (_sync)
-            wait_until_sent();
+            wait_until_acked();
 
         return _written;
     }