fixes for WiFiClient::write(Stream) (#7987)

fixes for WiFiClient::write(Stream) and Stream transfers
- remove deprecated WiFiClient::write(Stream,size)
- fix and deprecate WiFiClient::write(Stream) to use Stream::sendAll instead of ::sendAvailable
- update ESP8266WebServer::streamFile to use file.sendAll(client) instead of client.write(file)
- remove stream dependence in ClientContext
- Stream::send(): honor timeout in all case, avoid short transfer when output is temporarily full
- example WiFiEcho: show sendAll and sendAvailable

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -226,7 +226,7 @@ public:
     size_t contentLength = 0;
     _streamFileCore(file.size(), file.name(), contentType);
     if (requestMethod == HTTP_GET) {
-      contentLength = _currentClient.write(file);
+      contentLength = file.sendAll(_currentClient);
     }
     return contentLength;
   }