build: compile take too much memory and might freeze the host machine

## Brief

@Shouren have made some test for that:
https://github.com/4paradigm/OpenMLDB/pull/870#discussion_r766343266

turns out compile may easy to down if
- host memory is low, say 4G
- enable parallel build, current default to `$(nproc)`

especially compile targets like `avg_by_category_def.cc`

## Other

Should documented minimal requirements of compilaion

Look at the build configuration, likely in CMakeLists.txt, to understand the parallel build settings. Examine the problematic compilation target 'avg_by_category_def.cc' and similar files to gauge memory usage. The goal is to adjust build parallelism or document minimal memory requirements to prevent host freezes during compilation.

索引モデルが issue の本文から書いたものです。