Increase Telegram polling period to 20 s

[denis-stepanov]

Currently we poll Telegram every 10 s. However, the default timeout for SSL connection is currently hardcoded in ESP Core to 15 s. Due to this, if there is a temporary problem with connection to Telegram, polls might become overlapping (because called via Ticker) and piling up, eventually leading to a crash. The solution would be to make timeout 20 s instead of 10. In this application it is not critical to have fast response from a Telegram bot.

Note that there is a fix in Core esp8266/Arduino#8889 making the timeout configurable. If that would be available, one can keep polling period at 10 s and make timeout something like 5 s instead of 15 s. A healthy Telegram poll takes about 2 s.