feat(chain)!: `KeychainTxOutIndex` uses a universal lookahead

feat(chain)!: `KeychainTxOutIndex` uses a universal lookahead

The 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.

Instead of simply patching it for wallet, we alter `KeychainTxOutIndex`
to have a default lookahead value. Additionally, instead of a
per-keychain lookahead, the constructor asks for a `lookahead` value.
This avoids the footguns of having methods which allows the caller the
decrease the `lookahead` (and therefore panicing). This also simplifies
the API.

Co-authored-by: Antoine Poisot <darosior@protonmail.com>
Co-authored-by: 志宇 <hello@evanlinjin.me>