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.

https://github.com/nervosnetwork/ckb-light-client/blob/0a8c34ab0db24c8d3af53a894efb1a443ea7cae4/src/protocols/light_client/mod.rs#L364-L376

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 :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/nervosnetwork/ckb-light-client/pull/178/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/nervosnetwork/ckb-light-client/pull/178/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork) | `72.46% <100.00%> (+1.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.