Extension Context

[kpelzel]

Problem:
Some extensions are only discovered by the parent node. Currently if the parent node makes a discovery event after the node has gone into sync state, the next incoming hello packet will overwrite all extensions regardless of anything the parent node discovers. This is due to the source of truth for discoverable state coming from the child nodes.

Solution:
Here's a proposal for a simple solution. Each extension now has a "Context" method which is either:

• lib.ExtensionContext_PARENT
• lib.ExtensionContext_CHILD

If an extension has a context of PARENT, that extension will be removed from any hello packets that are coming from the child nodes because that extension can only be discovered by the parent. Anything with a child context will not be removed.

(Also, ignore my terrible commit messages)

[jlowellwofford]

We may want to be able to specify the context on a per-variable level. This would probably require the ability to add metadata to variables, which adds a bit of complexity, but could be used for other things as well.

Needs further discussion.

[jlowellwofford]

I would like to revisit this. While it may be nice to have per-variable support, I don't think that excludes per-extension support.

@kpelzel would you mind rebasing this to a modern commit and making sure it still works?