Merge bitcoindevkit/bdk#575: Remove database flush

5ff8320e3b1cb4e3971549a3e3c168f20762d04b add private function ivcec_to_u32 in keyvalue (KaFai Choi)
e68d3b9e63914c1008e1a4bb20a847428659d52e remove Database::flush (KaFai Choi)

Pull request description:

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  ### Description

  This PR is to remove Database::flush. See this issue for detail https://github.com/bitcoindevkit/bdk/issues/567

  ### Notes to the reviewers
  The 2nd commit is a small refactoring of adding a new private ivec_to_u32 to avoid too much code duplication. Please let me know if it's ok to include this in this PR or I should make it into a separate PR

  Currently existing test cases are shared across for all Databaes implementation so I am not sure if we should add  specific test cases for keyvalue(Tree) for this auto-flush behaviour?(and I feel like it's more a implementation detail). Please let me know how should I proceed for test case in this PR

  ### 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
  * [x] I've updated `CHANGELOG.md`

  #### Bugfixes:

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

ACKs for top commit:
  danielabrozzoni:
    re-ACK 5ff8320e3b1cb4e3971549a3e3c168f20762d04b

Tree-SHA512: eb37de8217efeb89d3ae346da36d0fb55aa67554d591b4759500f793bcf6aa7601c3d717fd473136c88e76aa72dbb6008ecf62b1d4ccf5ba3cbd1598f758522a

diff --cc CHANGELOG.md
Simple merge