refactor(queue/mysql): immutable log, delivery state, heartbeat store, fair leasing

[behinddwalls]

Summary

• Add delivery state store for tracking per-consumer message state separately from the immutable log
• Add subscriber heartbeat store for liveness detection and fair partition leasing
• Refactor message store to append-only immutable log with contiguous watermark tracking
• Refactor partition lease store with heartbeat-based fair leasing
• Add batch query support for offset and delivery state operations
• Add GC throttling and dead code removal
• Fix make fmt to skip read-only pkg/mod directory

Test Plan

Issues

[sbalabanov]

I think overall error processing and expectations on reliability of the heartbeats need to be better articulated. Both contracting the interface with comments and readme.

[behinddwalls]

I think overall error processing and expectations on reliability of the heartbeats need to be better articulated. Both contracting the interface with comments and readme.

I need to update those, I will do them in a separate PR.