feat: enable concurrent write

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

1. set DEFAULT_WRITE_CONCURRENT to 8.
2. Enable concurrent write for Copy To and ParquetWriter.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (2bf4b08) 85.82% compared to head (9519deb) 85.44%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3214      +/-   ##
==========================================
- Coverage   85.82%   85.44%   -0.38%     
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  Files         840      844       +4     
  Lines      137704   138639     +935     
==========================================
+ Hits       118183   118463     +280     
- Misses      19521    20176     +655     

[killme2008]

Good job! Could we provide some benchmark results for this feature? And I wonder if the default value of 8 is the best practice for most common usage.

[WenyXu]

Good job! Could we provide some benchmark results for this feature? And I wonder if the default value of 8 is the best practice for most common usage.

I will create a benchmark with throughput and latency in the different write concurrency settings. BTW, Based on some simple experiments I did before, a 10Gpbs bandwidth EC2 instance, setting the write concurrency to 25, the program could reach throughput: 500MiB/s with p50 ~250ms.

[killme2008]

LGTM