TCPListener issues

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1. In TCPListener.h: documentation for methods accept() and reject() is incomplete. _onIncoming is also not documented.
2. In TCPListener.cpp: In the implementations for start_listening() and stop_listening() should check that _socket.api is not NULL before invoking.

[ciarmcom]

ARM Internal Ref: IOTSFW-1603

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@bremoran bump point 1 above