API: show channel's partner state (active, inactive)

[kilrau]

Abstract

For our use case we'd need to know the detailed channel's partner state as follows:

• active: partner is online
• inactive: partner is offline

Motivation

For xud to know when to use a channel.

Specification

N/A

Backwards Compatibility

N/A

[kilrau]

As per #4620, it would be nice to take the state as argument and list all channel identifiers with that state.

[kilrau]

It would be great if pending channels came with a tx_id. Further, we'd need a stream to subscribe to channel events to avoid hitting the api/v1/channels endpoint every second or so. I opened a separate issue for that: #4756

[rakanalh]

@kilrau I took the liberty to update the issue and separate the context into this one and #4620

[karlb]

User presence can be queries from the PFS via API.