Merge bitcoindevkit/bdk#1736: Sqlite - allow persisting anchor without tx

18f5f3fa21f5e83dab9e30a5f536e5526883d580 test(sqlite): test persisting anchors and txs independently (valued mammal)
297ff3414909ae42d34c31f5c942fc81aa1d60f9 test(chain): add test `insert_anchor_without_tx` (valued mammal)
e69d10e7d387ffc96876b4290131d8fa9d4dd8ff doc(chain): document module `rusqlite_impl` (valued mammal)
8fa899b74619b549e4880c28a0f1d348cb68f755 fix(chain): Sqlite - allow persisting anchor without tx (志宇)

Pull request description:

  ### Description

  Previously, we may error when we insert an anchor where the txid being anchored has no corresponding tx.

  closes #1712
  replaces #961

  ### Notes to the reviewers

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  of the PR were done in a specific way -->

  ### Changelog notice

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  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### 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:
  evanlinjin:
    ACK 18f5f3fa21f5e83dab9e30a5f536e5526883d580
  ValuedMammal:
    ACK 18f5f3fa21f5e83dab9e30a5f536e5526883d580

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