Define a 'sub-channels' NETCONF capability

Using SSH transport for NETCONF has the ability to multiplex multiple channels inside a single SSH connection: https://tools.ietf.org/html/rfc4254#section-5.

This allows us to have multiple NETCONF sessions hosted in a single connection, taking advantage of NETCONF server capabilities, such as candidate commit conflict resolution.

Define a NETCONF capability, which advertises that the server can open multiple NETCONF sessions on the same SSH connection and implement it in both SB and NB plugins:

• the NB plugin should advertise the capability and react to new channels being open on the SSH connection
• the SB plugin should allow opening a new session on an existing connection if the device advertises the capability