Merge bitcoindevkit/bdk#652: Fix: Hang when `ElectrumBlockchainConfig::stop_gap ...

612da165f8cfbc2b0aee0a37b2cdc44d6da017c8 `Blockchain` stop_gap testing improvements (志宇)
8a5f89e129d421a41af02ea85383d5b82f5ff665 Fix hang when `ElectrumBlockchainConfig::stop_gap == 0` (志宇)

Pull request description:

  * Ensure `chunk_size` is > 0 during wallet sync.

  * Slight refactoring for better readability.

  * Add test: `test_electrum_blockchain_factory_sync_with_stop_gaps`

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->

  ### Description

  `Wallet::sync` hangs indefinitely when syncing with Electrum with `stop_gap` set as 0.

  The culprit is having `chunk_size` set as `stop_gap`. A zero value results in syncing not being able to progress.

  Fixes #651

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

  ~* [ ] This pull request breaks the existing API~
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    ACK 612da165f8cfbc2b0aee0a37b2cdc44d6da017c8

Tree-SHA512: 56f1bff788855facc21856209922594cff9f639c5c58ecd180a0493322a75a564b72ded330ab0b6d6c90007ce859d2b8a5d2870d619bae5ddf9a3d64837f3753

diff --cc CHANGELOG.md
Simple merge