[WIP]nydus-image: modify compact subcommand

[newthifans]

Brief introduction
Modify command "nydus-image compact" to compress the small blob in the nydus image into a large blob

Basic Usage
nydus-image compact --bootstrap /path/to/nydus_bootstrap --compact-config /path/to/compact.json --backend-config /path/to/backend-config.json

[codecov]

Codecov Report

Merging #1489 (8d255a4) into master (767adcf) will increase coverage by 0.17%.
Report is 7 commits behind head on master.
The diff coverage is 21.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1489      +/-   ##
==========================================
+ Coverage   62.51%   62.69%   +0.17%     
==========================================
  Files         123      123              
  Lines       43148    43268     +120     
  Branches    43148    43268     +120     
==========================================
+ Hits        26975    27125     +150     
+ Misses      14870    14831      -39     
- Partials     1303     1312       +9     

Files	Coverage Δ
storage/src/meta/chunk_info_v1.rs	95.93% <100.00%> (+0.03%)	⬆️
src/bin/nydus-image/main.rs	0.77% <0.00%> (ø)
builder/src/compact.rs	79.40% <12.50%> (-1.16%)	⬇️

... and 6 files with indirect coverage changes

[adamqqqplay]

@newthifans Thanks for your contribution! Can you add some unit tests to cover your changes?