gNMI Connection Lifetime Discussion

The current implementation of nucleus.Gnmi has a gNMI connection attached to the struct. This causes several issues:

• The connection is established upon instantiation of the struct and assumed to be active over the struct's lifetime
• The effects of an unexpected loss of the connection is unclear but possibly resulting in a panic

We need to discuss, if we want to keep this approach or if we want to use per-call connections.

/cc