…dent requests

This makes it possible to have different kinds of *parent key*, to
update a dependent request. A dependent request waits for the parent key
to be set, before it can be acted upon; before, it could only be an
event id, because a dependent request would only wait for an event to be
sent. In a soon future, we're going to support uploading medias as
requests, and some subsequent requests will depend on this, but won't be
able to rely on an event id (since an upload doesn't return an
event/event id).

Since this changes the format of `DependentQueuedRequest`, which is
directly serialized into the state stores, I've also cleared the table,
to not have to migrate the data in there. Dependent requests are
supposed to be transient anyways, so it would be a bug if they were
transient.

Since a migration was needed anyways, I've also removed the `rename`
annotations (that supported a previous format) for the
`DependentQueuedRequestKind` enum.

…`, not a raw event

Changelog: The send queue will now store a serialized
 `QueuedRequestKind` instead of a raw event, which breaks the format.
 As a result, all send queues have been emptied.

Because the latest migration would clear events to-be-sent from the send
queue, the test now failed. It's been considered OK.

Notably, make it super clear what parameters are required to create the
attachment type, since the function doesn't consume the whole
`AttachmentConfig` for realz.