[venus] use too much memory when export snapshot

[cloudxin]

Checklist

• This is not a security-related bug/issue. If it is, please follow please follow the security policy.
• This is not a question or a support request. If you have any lotus related questions, please ask in the venus forum.
• This is not a new feature request. If it is, please file a feature request instead.
• This is not an enhancement request. If it is, please file a improvement suggestion instead.
• I have searched on the issue tracker and the venus forum, and there is no existing related issue or discussion.
• I am running the latest release, or the most recent RC(release canadiate) for the upcoming release or the dev branch(master), or have an issue updating to any of these.
• I did not make any code changes to venus.

Venus component

• venus daemon - chain sync
• venus auth - authentication
• venus gateway - chain service gateway
• venus miner - mining and block production
• venus sealer/worker - sealing
• venus sealer - proving(WindowPoSt)
• venus market - storage deal
• venus market - retrieval deal
• venus market - data transfer
• venus light-weight client
• venus JSON-RPC API
• venus messager - message management (mpool)
• Other

Venus Version

venus version
{
	"Version": "1.2.4+git.135232873.dirty"
}

Describe the Bug

./venus chain export --recent-stateroots=920 --skip-old-msgs snapshot.car

Logging Information

Exporting snapshot files takes too long and takes up a lot of memory. 128G systems have memory overflow problems.

Repo Steps

1. Run '...'
2. Do '...'
3. See error '...'
   ...

[hunjixin]

@cloudxin 导出内存还是比lotus多？

[hunjixin]

目前方案lotus和venus都很多。继续追踪这个issue意义不大。先重启解决吧，或者以后看看能不能通过https://github.com/filecoin-project/venus/issues/5182解决