chain: set DEFAULT_LOOKAHEAD to 25

[yukibtc]

Description

Change DEFAULT_LOOKAHEAD to 25, as suggested by @LLFourn in #1262.

Fixes #1262

Checklists

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• I've signed all my commits
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• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
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Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
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[LLFourn]

ACK 285ff46

[evanlinjin]

ACK 285ff46

I'm not a fan of this as a long term solution to solving slow startup times. However, I think it would be safe to assume that, for now, no user of BDK wallet requires a lookahead of >25.

We should do follow up PRs for some or all of the following:

1. Make lookahead configurable in bdk::Wallet.
2. Cache spks in persistence.
3. Figure out how to improve the performance of deriving spks.

[LLFourn]

Another idea is that we could set this value lower in debug compilations and higher in --release. Not sure if this would offend people though.