refactor: use set_lookahead in set_lookahead_for_all

[vladimirfomene]

Description

Use set_lookahead in set_lookahead_for_all.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[realeinherjar]

ACK: e89cf5a

set_lookahead does the same thing as what was replaced.

Check:

bdk/crates/chain/src/keychain/txout_index.rs

Lines 185 to 188 in c20a4da

	pub fn set_lookahead(&mut self, keychain: &K, lookahead: u32) {
	self.lookahead.insert(keychain.clone(), lookahead);
	self.replenish_lookahead(keychain);
	}

[evanlinjin]

ACK e89cf5a