operational-state (datastore) change notifications

[choppsv1]

Add framework for notifications sent to front-end clients for changes to operational state (datastore). This is to support various YANG push RFCs such as:

• RFC7923: Requirements for Subscription to YANG Datastores
• RFC8639: Subscription to YANG Notifications
• RFC8640: Dynamic Subscription to YANG Events and Datastores over NETCONF
• RFC8641: Subscription to YANG Notifications for Datastore Updates
• RFC8650: Dynamic Subscription to YANG Events and Datastores over RESTCONF

[donaldsharp]

What is the chance that abs_path and path are equal and path was passed in?

[choppsv1]

abs_path starts out assigned as NULL, it is then either assigned the value path or it is a new allocation from lyd_path(). path is not currently allowed to be NULL (notice we use path[0] without checking for path == NULL). So path can only ever equal abs_path when we set it that way (which is what our equality check is deciding, did we set it or newly allocate it).

[choppsv1]

That said, I don't see a reason we couldn't also support a NULL path for updating the path/value of the tree node itself, so I'll modify the code to also handle path == NULL.

[choppsv1]

actually nm, this gets tricky deeper in the implementation, i'll leave extended functionality for later if we need it.