Add request validating chain element for nsmgr

[denis-tingaikin]

Motivation

nsmgr could validate incoming requests from NSC to fast fail it if the incoming request is invalid. In this case, we can get a fast response without resource allocation and deleting in forwarders, endpoints.

Checks

1. Check that the incoming request has not selected a mechanism in the connection on the first request.
2. Check that the prefered mechanisms list is not empty.
3. TODO: add other checks

[denis-tingaikin]

@edwarnicke Does this element make sense?