net: lib: http_server: Close sockets on server core errors

In case of fatal errors (during poll() or when handling listening
socket), the server operation is restarted. It was missed however, that
sockets opened for the server should be closed in such case.

Additionally, in case there were active client connections, it's needed
to cleanup related resources, otherwise running timers may trigger a
crash.

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