introduce findById()

[staltz]

Context

In the new multi-format ssb-ebt we need to be able to get any feedID and infer what is it: is it a classic main feed? Is it a classic index feed? Is it a bendy butt? (The first 2 questions are the most important) It's not enough to just look at the feedId and infer the feedformat, we also need to know the feedpurpose. In ssb-replication-scheduler we will also need something like this.

Solution

New API findById(). Turns out it was already implemented as getMetadata, I just did further tweaking in its internals, to:

• If the feedId given to findById() has been tombstoned, return null
• Infer the feedformat from the feedId
• Remove msg.value.content stuff such as type and nonce etc
• Bundle unknown fields in msg.value.content under metadata object, like it is during subfeed creation

[arj03]

Great we can reuse the existing function and just extend it a bit. I have some comments on the usage in ebt, but I'll take them in another channel. This PR in itself is really good in that it makes this module quite a bit more general.