Read/write streamMessage.newGroupKey & enable key revocation

[timoxley]

• Adds support for reading and writing streamMessage.newGroupKey.
• Tests that revoking subscription permissions and rekeying actually removes subscribers from the stream.
• Expands subscription logging

This PR used to be on top of Persist Groupkeys PR #204 but that PR is going to take some more work. I've reworked this to be on top of master now.

[linear]

NET-208 Add revocation based on monitoring subscriber set

Melchior started on this, but it never got merged. https://github.com/streamr-dev/streamr-client-javascript/pull/130/files

[teogeb]

This method could be plural as it generates an array of GroupKeys?

[timoxley]

Hm, sort of, it could probably do with some docs though.

Basically it's giving you the current + (optional) next group key.

If there's two values, it'll encrypt the message with the first, but embed the second in the stream message (encrypted with the first), then the next time you call useGroupKey you will get that second value in the first position, and an empty value second position, until a new group key is assigned. When a new group key is assigned, it'll use the existing key one more time, embedding the next key, then start using the new key from there on.

Something like this:

setGroupKey(keyA)
useGroupKey() // => [keyA, undefined]
useGroupKey() // => [keyA, undefined]
setGroupKey(keyB)
useGroupKey() // => [keyA, keyB]
useGroupKey() // => [keyB, undefined]
useGroupKey() // => [keyB, undefined]
setGroupKey(keyC)
useGroupKey() // => [keyB, keyC]
useGroupKey() // => [keyC, undefined]
// etc

Where useGroupKey is called before encrypting each message.
Iff there's a second value, it will be set as the streamMessage.newGroupKey

[teogeb]

On possibility would be to introduce some helper data type, like GroupKeyQueue. It would encapsulate this e.g. max length handling.

[timoxley]

getGroupKey definitely has a confusing name now though, since it returns an array of keys, one of which should be the one you're after, or an empty array.