Implement Periodic Keychain Compaction to Optimize VSS Sync

[EthanYuan]

Problem:

The persisted bdk_wallet::ChangeSet (keychain) grows significantly over time, mainly due to local_chain history. This large size (more than 1 MB) hinders efficient synchronization with VSS.

Solution:

This PR implements periodic keychain compaction within the start_sync loop:

1. Checks the persisted keychain size each loop iteration.
2. If size exceeds KEYCHAIN_COMPACTION_SIZE_THRESHOLD_BYTES (256 KB):
   • The reason why it is not set to a smaller size, such as 128 KB or 64 KB, is that some super users, themselves, have a very large amount of unspent utxo. this optimal value needs to be further observed and adjusted.
   • Synchronously rebuilds the wallet state by creating a new Wallet instance and performing OnChainWallet::full_scan (using Esplora, stop_gap=50).
   • Persists the new wallet's state using storage.restore_changes, overwriting the old keychain. (restore_changes attempts backup).
   • Only if persistence succeeds, the in-memory wallet instance is replaced with the new one.

Test

• local tests

keychian compaction log（10700 Bytes just for test）：

2025-04-18 01:37:04.841 6bf4 INFO  [mutiny_core::onchain:882] Keychain size threshold exceeded 10700 Bytes, spawning simplified compaction task.

2025-04-18 01:37:04.843 6bf4 DEBUG [mutiny_core::onchain:948] PRE-COMPACTION size: 45044 bytes. Approx component sizes (bytes): LocalChain=44512, TxGraph=50, Indexer=88

2025-04-18 01:37:07.947 6bf4 DEBUG [mutiny_core::onchain:948] POST-COMPACTION size: 547 bytes. Approx component sizes (bytes): LocalChain=83, TxGraph=50, Indexer=20

2025-04-18 01:37:07.986 6bf4 INFO  [mutiny_core::onchain:927] Keychain compaction completed successfully.
2025-04-18 01:37:07.986 6bf4 INFO  [mutiny_core::onchain:928] Keychain compaction took 3 seconds

keychain backup：