app: Update socket AT commands to use handle-based API

All socket operations now require a handle obtained from #XSOCKET
or #XSSOCKET command. This allows more flexible way to manage
sockets.

Remove AT command #XSOCKETSELECT.
Add AT command #XCLOSE to close individual socket or all sockets
at once.

Update documentation.

SLM-50

Signed-off-by: Juha Ylinen <juha.ylinen@nordicsemi.no>

Author: juhaylinen