Merge bitcoindevkit/bdk#1874: fix(wallet): allow PersistedWallet to be Send + Sync

14251a43be73796f0db98cee8a0b5e88e4b4c51a fix(wallet): allow PersistedWallet<P> to be Send + Sync even if P is !Sync (Steve Myers)

Pull request description:

  ### Description

  fixes #1873 based on solution proposed by @praveenperera .

  ### Notes to the reviewers

  This is not a breaking change since it's only changing the internal `_marker` field's type.

  ### Changelog notice

  - Fix PersistedWallet to be Send + Sync, even when used with a !Sync persister type such as rusqlite::Connection.

  ### 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:
  ValuedMammal:
    ACK 14251a43be73796f0db98cee8a0b5e88e4b4c51a

Tree-SHA512: 8ce8ad7d9b2d962114e225074b485bc91671164fe2f2fcd59f48a6c4f53a885bb215d0017584292629fa02b5ca19a67a40a7855fcd4c2490fefd2b3be65f717f