SPV: Sync state save during filter sync taking 15+ seconds each time

[pauldelucia]

It's slowing sync down a lot. Some saves take over a minute.

2025-11-27T08:55:49.021509Z  INFO dash_spv::client::status_display: 📊 [SYNC STATUS] Headers: 2379095 | Filter Headers: 2379095 | Filters: 1230034 | Latest ChainLock: #2379108 | Filters Matched: 300 | Blocks w/ Relevant Txs: 0 | Blocks Processed: 297 | Balance: 0 BTC
2025-11-27T08:56:26.319603Z DEBUG dash_spv::storage::disk::state: Saved sync state at height 2379095
2025-11-27T08:56:26.320513Z DEBUG dash_spv::client::sync_coordinator: Saved sync state: headers=2379095, filter_headers=2379095, filters=1230034

Note the timestamps.

2025-11-27T08:54:57.576119Z  INFO dash_spv::client::status_display: 📊 [SYNC STATUS] Headers: 2379095 | Filter Headers: 2379095 | Filters: 1230033 | Latest ChainLock: #2379108 | Filters Matched: 300 | Blocks w/ Relevant Txs: 0 | Blocks Processed: 297 | Balance: 0 BTC
2025-11-27T08:55:43.866653Z DEBUG dash_spv::storage::disk::state: Saved sync state at height 2379095
2025-11-27T08:55:43.867606Z DEBUG dash_spv::client::sync_coordinator: Saved sync state: headers=2379095, filter_headers=2379095, filters=1230033

[xdustinface]

The issue here is the ChainWork calculation. We currently calculate the work of all stored headers every time we save the state. We should probably have a ChainWork object stored in the ChainState and update it incrementally with incoming headers.

[xdustinface]

Related code has been rewritten in #411