feat: get more last headers in the first proof of a newly connected peer

[yangby-cryptape]

Description

In the following scenario:

• Let $N$ denote the parameter LAST_N_BLOCKS.
• A light client L is shutdown with last state $s$.
• Then, restart L, and it connects to a peer P whose tip is $s+t$ and $t < N$.
  So, the first proof that L received from P, will contains only $t$ last headers.

If $t$ is very small, and P changes to another fork chain, then the fork block will be hard to find.

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

Lines 364 to 376 in 0a8c34a

	let fork_number = reorg_last_headers.iter().rev().find_map(|reorg_header| {
	let number = reorg_header.number();
	old_last_headers
	.get(&number)
	.map(|hash| {
	if &reorg_header.hash() == hash {
	Some(number)
	} else {
	None
	}
	})
	.unwrap_or_default()
	});

As a result, it is easy to trigger "long fork detected".

Changes

• Find the largest header from the last headers in the storage, which satisfies that $s - x > N$ ($x$ is the number of that header).
• If not found any header in the above step, then choose the smallest header from the last headers in the storage.
• Use the header which is found in above steps as the start header.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0a8c34a) 71.42% compared to head (0c653da) 72.46%.
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #178      +/-   ##
===========================================
+ Coverage    71.42%   72.46%   +1.04%     
===========================================
  Files           25       25              
  Lines         6512     6523      +11     
===========================================
+ Hits          4651     4727      +76     
+ Misses        1861     1796      -65     

Flag	Coverage Δ
unittests	72.46% <100.00%> (+1.04%)	⬆️

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