Use long polling

[sanderkob]

What is needed to implement long polling? Simply changing the timeout value in

AsyncTelegram2/src/AsyncTelegram2.cpp

Line 138 in e48c5a9

snprintf(payload, BUFFER_SMALL, "{\"limit\":1,\"timeout\":0,\"offset\":%" INT32 "}", m_lastUpdateId);

does not work, at least not in my setup.
I was expecting this to work, since the connection from the bot to the server is only closed when instructed by the server.