Setting electrum client's stop_gap as 0 results in hang

[evanlinjin]

Describe the bug

Setting ElectrumBlockchainConfig::stop_gap as 0 results in infinite loop during wallet setup/sync.

bdk/src/blockchain/electrum.rs

Line 302 in 063d51f

pub stop_gap: usize,

To Reproduce

• Create wallet with ElectrumBlockchainConfig::stop_gap set as 0
• Call Wallet::sync

Expected behavior

• Electrum blockchain should still be functional with stop_gap == 0

Build environment

• BDK tag/commit: v0.19.0 063d51f
• OS+version: Fedora Linux 5.18.5-200.fc36.x86_64
• Rust/Cargo version: cargo 1.62.0 (a748cf5a3 2022-06-08)
• Rust/Cargo target: x86_64-unknown-linux-gnu

Additional context

Example code:

let mut test_client = TestClient::default();

let factory = Arc::new(
    ElectrumBlockchain::from_config(&ElectrumBlockchainConfig {
        url: test_client.electrsd.electrum_url.clone(),
        socks5: None,
        retry: 0,
        timeout: None,
        stop_gap: 0, // STOP_GAP IS ZERO
    })
    .unwrap(),
);

let wallet = Wallet::new(
    "wpkh(L5EZftvrYaSudiozVRzTqLcHLNDoVn7H5HSfM9BAN6tMJX8oTWz6)",
    None,
    bitcoin::Network::Regtest,
    MemoryDatabase::new(),
)
.unwrap();

// HANGS HERE INDEFINITELY!!!
factory
    .sync_wallet(&wallet, None, Default::default())
    .unwrap();

[LLFourn]

As I mentioned in the PR I am not sure what the semantics of stop_gap = 0 should be. It definitely shouldn't hang though.