add mechanism for updating client schema context

[michalvasko]

It is possible for server to modify its supported schemas (YANG 1.1 RFC) and the client may also want to support the new context. libnetconf2 does not allow this currently.

The implementation can add one new function (*_update), which would compare its current module-set-id with the one received from the server and if they would not match, try to update the context. If an application required it already, it can implement this completely outside libnetconf2 (it can access the context). Nevertheless, libnetconf2 already implements several ways of retrieving schemas, at least some of which the application would duplicate.