High-priority requests support

[Otaiki1]

Context

Some raffles (e.g. flash raffles) need faster reveals.

Goal

Add priority levels to the Bull queue.

Contributor guide

1. Directory: oracle/
2. Map high-stakes (> 500 XLM) to higher priority by default.
3. Support 'priority' flag in contract event (optional).
4. Worker processes high-priority queue first.
5. Monitor SLA for high-priority jobs.

References

• Bull queue priority documentation.

[Hunter-baddie]

@Hunter-baddie has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

can i work on this issue?

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