fix: let peers cached their own last headers to avoid be overridden

[yangby-cryptape]

Description

At present, only one set of verified last N block headers are cached.

ckb-light-client/src/protocols/light_client/peers.rs

Lines 23 to 24 in 0a8c34a

	// verified last N block headers
	last_headers: RwLock<Vec<HeaderView>>,

But, in fact, all peers could have different sets of last N block headers.

Consider that:

• A and B are same chain.
• A's tip is N.
• B's tip is N+k.
• Light client connects to A and B at same time.
• The last state proof from B is arrived earlier than A's.

What will be happened?

• B's last N headers are cached.
• Then, A's last N headers overrides B's.

How to fix that?

Traverse last headers of all peers every time.

Commits

• test: reproduce the bug that last headers could be overridden by worse data
• fix: let peers cached their own last headers to avoid be overridden