Fixes three minor issues.
Issue #1: Disallow sink node from using sendToSink() method.
Issue #2: Allows users to use discoveryREQ() by making it a public method. Users can now use discoveryREQ() when needed. Although usage is not needed, in some rare scenarios, in case the user needs, it may be used.
Issue #6: Minor compatibility issues for ESP8266 Node MCU.
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