fix: Unsubscribe required on exit

[simlecode]

关联的Issues (Related Issues)

改动 (Proposed Changes)

这两处退出时没有取消订阅，这样会导致同步卡住：

https://github.com/filecoin-project/venus/blob/master/pkg/chain/store.go#L706 和 https://github.com/filecoin-project/venus/blob/master/pkg/chain/store.go#L713

附注 (Additional Info)

日志：

2023-08-16T17:22:30.357+0800	INFO	chain.store	chain/store.go:545	SetHead { bafy2bzacea6lern2svj7b56e37h43gijg5xde2kaufsqxe4yrf3x3cbdpyl6e } 3129045
2023-08-16T17:22:30.358+0800	INFO	chainsync.syncer	syncer/syncer.go:263	handle tipset height 3129045, count 1, took 0.1558(s)
2023-08-16T17:22:30.358+0800	ERROR	chain.store	chain/store.go:712	closing head change subscription due to slow reader

2023-08-16T17:38:00.227+0800	INFO	consensus	consensus/block_validator.go:142	block validation	{"took": 0.063988209, "height": "3129076", "block": "bafy2bzaceavo5dysax2deig4mpqxhbeqzgxfxi4v3t3swqbx3pcuvomnot4rs", "age": 0.227459898}
2023-08-16T17:38:00.229+0800	INFO	chain.store	chain/store.go:545	SetHead { bafy2bzaceavo5dysax2deig4mpqxhbeqzgxfxi4v3t3swqbx3pcuvomnot4rs } 3129076

从日志上看在 3129045 高度，chainnotify 退出了，然后同步卡在 3129076，相差 31 个高度，与 chainnotify 的 channel buffer(32) 容量接近。

venus-0817.log.tar.gz

自查清单 (Checklist)

在你认为本 PR 满足被审阅的标准之前，需要确保 / Before you mark the PR ready for review, please make sure that:

• 符合Venus项目管理规范中关于PR的相关标准 / The PR follows the PR standards set out in the Venus project management guidelines
• 具有清晰明确的commit message / All commits have a clear commit message.
• 包含相关的的测试用例或者不需要新增测试用例 / This PR has tests for new functionality or change in behaviour or not need to add new tests.
• 存在兼容性问题（接口, 配置，数据，灰度），如果存在需要进行文档说明 / This PR has compatibility issues (API, Configuration, Data, GrayRelease), if so, need to be documented.
• 包含相关的的指南以及文档或者不需要新增文档 / This PR has updated usage guidelines and documentation or not need
• 通过必要的检查项 / All checks are green

[codecov-commenter]

Codecov Report

Merging #6103 (5f219aa) into master (254a9cb) will decrease coverage by 1%.
The diff coverage is 83%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #6103   +/-   ##
======================================
- Coverage      24%     24%   -1%     
======================================
  Files         662     662           
  Lines       80383   80389    +6     
======================================
- Hits        19700   19685   -15     
- Misses      58075   58091   +16     
- Partials     2608    2613    +5     

[LinZexiao]

LGTM