fix: allow no matched blocks for samples in the range of boundary block

[yangby-cryptape]

Issue

Assume the environment is as following:

• Total difficulty for blocks 0~4 are [1, 10, 20, 30, 40] and for blocks 5~9 are [45, 46, 70, 80, 90] .
• The last block number is 10.
• The last-n-blocks is 3.
• The difficulty boundary is 60.
• The difficulties samples are [10, 20, 30, 50, 51, 52].

So the server should reply proof for blocks [1, 2, 3, 7, 8, 9] to the client.

But the client will raise an error because there is no matched block for 50 and 50 is not in the difficulty range of those continuous blocks.

How to Fix

Before this PR, the check for difficulties without matched blocks is:

• If there exists difficulties without matched blocks, there should be no blocks between the last sampled block and the boundary block.

After this PR, change the check to be:

• The difficulties without matched blocks should be greater than the total difficulty of the boundary block's parent.

Commits

• test: add a test for a bug and add logs for debugging
• fix: allow no matched blocks for samples in the range of boundary block

p.s. Review the last commit first is easier because the code for fix is only 1 lines, half code for logs and others for a test.

[codecov-commenter]

Codecov Report

Base: 76.75% // Head: 75.99% // Decreases project coverage by -0.76% :warning:

Coverage data is based on head (72ac841) compared to base (d1e670d). Patch coverage: 31.88% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #80 +/- ## =========================================== - Coverage 76.75% 75.99% -0.77% =========================================== Files 23 23 Lines 4208 4265 +57 =========================================== + Hits 3230 3241 +11 - Misses 978 1024 +46 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `75.99% <31.88%> (-0.77%)` | :arrow_down: | 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. | [Impacted Files](https://codecov.io/gh/nervosnetwork/ckb-light-client/pull/80?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork) | Coverage Δ | | |---|---|---| | [...s/light\_client/components/send\_last\_state\_proof.rs](https://codecov.io/gh/nervosnetwork/ckb-light-client/pull/80/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork#diff-c3JjL3Byb3RvY29scy9saWdodF9jbGllbnQvY29tcG9uZW50cy9zZW5kX2xhc3Rfc3RhdGVfcHJvb2YucnM=) | `84.78% <31.88%> (-6.51%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork)

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