Define and implement a 'session-candidate' NETCONF extension

Base RFC6241 does not provide enough discovery services to understand how the candidate data store works with respect to other sessions and recommends locking the candidate.

This does not align well with how the ODL data store works, specifically ODL does not have a shared candidate store, but rather implements it using MVCC, where each session can have a candidate and conflicts are resolved on commit.

Unfortunately this capability is not discoverable, which means that ODL NETCONF SB plugin cannot take advantage of it when connecting to and ODL-based device.

Define a new NETCONF capability, which indicates that the candidate store is not shared and exists only as long as the session does.

Implement this capability on the NB and make the SB plugin recognize it, so it does not lock the candidate store.