chore: deny unused results

[MichaelScofield]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

The rustc lint: "deny unused-results" can check whether a return value is handled or not.

Pros to this lint: force people to deal with all return values, reduce the possibility of lurked bugs. Cons: sometimes could be annoying, even the most trivial return values (HashMap::insert for example) have to be dealt with.

According to our internal poll to date (positive 4, neutral 4, negative 0), we decide to enable it.

This PR goes through all our codes and fix all the warnings. Mainly:

• use let _ = ...
• use HashMap::from where possible
• assert!(result.is_ok()) instead of result.unwrap()

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #1825 (4e9a76f) into develop (dcfce49) will decrease coverage by 0.30%.
The diff coverage is 88.94%.

@@             Coverage Diff             @@
##           develop    #1825      +/-   ##
===========================================
- Coverage    86.51%   86.22%   -0.30%     
===========================================
  Files          587      587              
  Lines        95642    95607      -35     
===========================================
- Hits         82745    82434     -311     
- Misses       12897    13173     +276     

[evenyag]

This lint is too verbose. Could we add #[must_use] to methods that need this check instead?

https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute

[evenyag]

• assert!(result.is_ok()) instead of result.unwrap()

I still prefer using unwrap() as it yields the error message. Assertion on is_ok() only tells us there is an error.

[killme2008]

LGTM