net: telnet: Switch TELNET shell backend to use sockets

Rework TELNET shell backend to use socket API for communication and
socket service library for socket monitoring.

Additionally, rework the TX part so that non-blocking TX is used when
sending from the system work queue. In case transfer is not possible at
the moment, the TX work is rescheduled instead of blocking the system
work queue.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>