`lightning-liquidity` Persistence Tracking Issue

[tnull]

• Prefactors: lightning-liquidity: Pre-/Refactors to prepare for persistence #4008
• Add service handler and event queue serialization logic: lightning-liquidity persistence: Add serialization logic for services and event queue #4059
• Add boilerplate to account for KVStore and KVStoreSync variants: lightning-liquidity persistence: Add serialization logic for services and event queue #4059
• Wire up background processor to drive general persistence
• Avoid re-persisting peer states if no changes happened (needs_persist flag everywhere)
• Persist in-line for very important calls that need to be persisted before we return
• Add roundtrip test coverage
• Look into parallizing persistence (e.g., we could persist batches of peer states at the same time, instead of persisting them sequentially)