feat: Implement stateful cursor

[scarmuega]

Problem Statement

When running in daemon mode, a restart of the process will make Oura start crawling from the initially configured point. Sinks should implement idempotency when processing an event, but depending on the use-case, restarting might generate undesirable lags and waste of resources.

Feature Scope

A "cursor" framework will be implemented such that it is agnostic of any concrete source, filter or sink. This framework will resemble a very simple key/value store. A sink stage would use the cursor framework to persist the point of the chain at certain "checkpoint" events. A source sink would use the cursor framework to check the last persisted checkpoint before starting to pull data from the node.

[gitmachtl]

is it possible to enable/disable that?

[scarmuega]

@gitmachtl it will be an opt-in feature for the daemon mode. You will have to enable it via config. It is meant for sinks such as Elasticsearch and Rotating Logs. The watch and dump command will keep working as they currently do.