Storage for Thread/changes

[chibenwa]

Implement a ThreadChangeRepository (API / memory / Cassandra)

[chibenwa]

Speaking with Jarry from AERC today I realise storage for Thread/changes is not needed. We can easily implement this as off today!

Here is how:

GETTING state for threads -> get the state for email ;-)

When I ask for changes since thread state B
THEN I just list Email changes since that date
AND we aggregate the data

addedEmailIDs = union(emailState.added.id)
addedTHreadCandidates = addedEmailIDs.map(this::retrieveThread) // record Thread(id, emailIds)
addedTHread = addedTHreadCandidates.filter(_.emailIds all contained in  addedEmailIDs)


changedEmailIDs = union(emailState.added.id ++ emailState.removed.id ) // updated emails never impact threads
updatedThreads = changedEmailIDs.map(this::retrieveThread falback to threadId is messageId) - addedTHread

This will effectively return deleted threads as 'updated' but clients WILL be able to resynchronise this and detect the actual deletion through a Thread/get call.

Cc @quanhoang1408