Merge bitcoindevkit/bdk#1229: Use a universal lookahead value for `KeychainTxOutIndex...

1def76f1f17fc876534c798883099c58de57c0fd chore: make clippy happy and bump clippy msrv (志宇)
c9467dcbb27bd497006846dcfcefb0c2cf8823c4 chore: improve documentation of lookahead (LLFourn)
bc796f412acdc3d2cd96f7aec0f24fa47c1fe889 fix(example): bitcoind_rpc_polling now initializes local_chain properly (志宇)
4fd539b6470f7f771e3b5e09e3287952fa7a1825 feat(chain)!: `KeychainTxOutIndex` uses a universal lookahead (Antoine Poinsot)

Pull request description:

  ### Description

  The `bdk::Wallet` is currently created without setting any lookahead value for the keychain. This implicitly makes it a lookahead of 0. As this is a high-level interface we should avoid footguns and aim for a reasonable default.

  To fix this, we have also decided to change `KeychainTxOutIndex` to have a default lookahead. Additionally, we have simplified the `KeychainTxOutIndex` API to have a single `lookahead` that is ONLY set at construction `KeychainTxOutIndex::new(lookahead: u32) -> Self`. This avoids the footguns of having methods which allows the caller to decrease the `lookahead` (which will panic).

  ### Notes to the reviewers

  ~A way to set this value externally is introduced in #1172. This PR only aims to use a saner default than 0. `1_000` is the value used by the Bitcoin Core wallet, and that seems reasonable to me.~

  Edit: we should NOT allow setting the `lookahead` value after-the-fact. Instead, the `lookahead` should be provided to the wallet's constructor.

  @evanlinjin: I don't think additional tests are necessary as no additional features are added, and the surface area of the API is decreased. The original tests already thoroughly test the `lookahead` concept.

  ### Checklists

  #### All Submissions:

  *(This section was updated as this PR changed from being a simple setting to introducing a new method.)*

  * [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
  ~* [ ] I've added tests~
  * [x] I've added docs

ACKs for top commit:
  LLFourn:
    ACK 1def76f1f17fc876534c798883099c58de57c0fd

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