perf(mito): scan SSTs and memtables in parallel

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR scans SSTs and memtables in parallel. Scanning each file or memtable is still sequential.

The implementation of parallel scan is quite intuitive: It spawns a scan task for each data source and yields the scanned batch to the merge reader via a channel.

There are two new options for parallel scan.

• scan_parallelism: The default parallelism is 1/4 of CPU cores
• parallel_scan_channel_size: The default channel size is 32

TSBS results

query type	parallelism 1 avg (ms)	parallelism 4 avg (ms)
cpu-max-all-1	55.80	27.89
cpu-max-all-8	202.82	114.03
double-groupby-1	1144.48	469.67
double-groupby-5	2650.60	883.85
double-groupby-all	4411.23	1589.75
groupby-orderby-limit	1507.01	502.63
high-cpu-1	52.65	24.80
high-cpu-all	8058.48	5484.44
lastpoint	14225.00	7393.31
single-groupby-1-1-1	20.81	12.80
single-groupby-1-1-12	34.29	13.47
single-groupby-1-8-1	68.47	42.45
single-groupby-5-1-1	28.22	16.57
single-groupby-5-1-12	42.64	19.21
single-groupby-5-8-1	94.53	61.04

Larger parallelism might not always help as the total number of SSTs to scan might be less than the parallelism.

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)

• Support parallel scan in mito engine #2806

[codecov]

Codecov Report

Merging #2852 (f1210b8) into develop (f9e7762) will decrease coverage by 0.30%.
Report is 10 commits behind head on develop.
The diff coverage is 95.41%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2852      +/-   ##
===========================================
- Coverage    84.73%   84.44%   -0.30%     
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  Files          749      754       +5     
  Lines       117900   118785     +885     
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+ Hits         99900   100303     +403     
- Misses       18000    18482     +482     

[fengjiachun]

LGTM