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
