client CHANGE using get-schema operation on ietf-yang-library
client CHANGE using get-schema operation on ietf-yang-library

Use get-schema to get ietf-yang-library instead of using internal one.
It allows to handle situation when client-side libyang internally
implements different version of ietf-yang-library.

Fixes CESNET/Netopeer2GUI#4

CESNET/libnetconf2,CESNET/libnetconf2