Decentralization Client: Implement ice-box for out-of-order messages

[mkysel]

Is your feature request related to a problem?

When a client subscribes to the decentralized backend, it might receive messages out of order.

Group messages can be inherently "out of order" and can be sorted by timestamp, but messages also depend on commits in their "depends_on" field.

A message can not be processed before all messages in the "depends_on" field have been processed.

This does not mean that other messages that have different dependencies can't proceed.

As such, the client must implement an "ice-box" that holds these messages until the dependencies arrive.

There are two solutions I can think of:

• either the client persists the ice-box and moves the last_seen cursor. Processing these messages across restarts.
• or it keeps the ice-box in memory, but might have to re-pull if a restart happens

Describe the solution to the problem

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Describe the uses cases for the feature

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Additional details

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