Merge bitcoindevkit/bdk#579: Faster sync by collecting esplora ureq thread handles

adef166b22d5c95006df39fd8c57574624071189 Create vector of thread handles to spawn threads (nickfarrow)

Pull request description:

  ### Description
  Speeds up esplora ureq syncing. Taken from https://github.com/bitcoindevkit/bdk/pull/560

  The current sync just creates a map of scripts to joinhandles which doesn't yet spawn the sync threads due to lazy evaluation. In the following `handles.map()`, the thread handles are *sequentially* evaluated and joined. With the fix, the handles are collected so that the threads spawn in parallel, and then joined

  ### Notes to the reviewers

  I had to add a `#[allow(clippy::needless_collect)]` so that it wouldn't complain about collecting and then iterating. (Perhaps clippy is partially responsible for this issue!)

  Tested sync performance by doing a fresh sync on an existing [gun](https://gun.fun) wallet.
  ```
  ---- Before fix ---
  real0m13.121s
  real0m13.367s
  real0m13.211s

  ---- After fix ----
  real0m5.516s
  real0m5.251s
  real0m5.594s
  ```

  ### 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
  * [ ] 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:
  notmandatory:
    ACK adef166b22d5c95006df39fd8c57574624071189

Tree-SHA512: 47c617117afde9b4706bfa63759bf06d1ec60ff95d8a80931e5b7e40e3293c855d2f7dac0c681173d43aecf77201a842e739b82291da09ac81909cf526a51c8d