* StreamConstPtr: prevent from passing a temporary String instance
* unconditionally allow progmem chars
* missing virtual destructor in Stream
(warning: deleting object of abstract class type 'Stream' which has non-virtual destructor will cause undefined behavior [-Wdelete-non-virtual-dtor])
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
