refactor: make all quota implementation logic clear

[allencloud]

Signed-off-by: Allen Sun allensun.shl@alibaba-inc.com

Ⅰ. Describe what this PR did

Once the disk quota implementation code is not well-mantained. This could be the potential risk in the project. Here we need to try to make the interface, variable, code block much more clear and readable.

Then in this PR, I try to refactor to polish the disk quota implement:

1. add more annotations for the interface, variables, and some code procedure(add example), so that I think unit test case could be added much more easily.
2. add a simple unit test for that, and more will be in the future.
3. encapsulate some code into common part like loadQuotaIDs.

Ⅱ. Does this pull request fix one issue?

none

Ⅲ. Describe how you did it

none

Ⅳ. Describe how to verify it

none

Ⅴ. Special notes for reviews

none

[codecov-io]

Codecov Report

Merging #1736 into master will increase coverage by 0.14%.
The diff coverage is 11.41%.

@@            Coverage Diff             @@
##           master    #1736      +/-   ##
==========================================
+ Coverage   63.55%   63.69%   +0.14%     
==========================================
  Files         200      201       +1     
  Lines       15527    15492      -35     
==========================================
  Hits         9868     9868              
+ Misses       4424     4388      -36     
- Partials     1235     1236       +1

Flag	Coverage Δ
#criv1alpha1test	33.86% <10.86%> (+0.03%)	⬆️
#criv1alpha2test	34.38% <10.86%> (+0.06%)	⬆️
#integrationtest	38.49% <11.41%> (+0.09%)	⬆️
#unittest	20.91% <1.63%> (-0.58%)	⬇️

Impacted Files	Coverage Δ
pkg/system/sysinfo.go	63.63% <ø> (ø)	⬆️
storage/quota/set_diskquota.go	9.09% <ø> (ø)	⬆️
daemon/mgr/container.go	54% <0%> (ø)	⬆️
storage/quota/grpquota.go	0% <0%> (ø)	⬆️
daemon/mgr/spec_hook.go	40.62% <100%> (-4.17%)	⬇️
pkg/system/device.go	100% <100%> (ø)
storage/quota/prjquota.go	9.65% <18.75%> (+1.56%)	⬆️
storage/quota/quota.go	17.96% <6.34%> (-1.33%)	⬇️
ctrd/image.go	78.81% <0%> (-2.47%)	⬇️
... and 2 more

[pouchrobot]

ping @allencloud

CI fails according integration system.
Please refer to the CI failure Details button to corresponding test, and update your PR to pass CI.

If this is flaky test, welcome to track this with profiling an issue.

build url: https://travis-ci.org/alibaba/pouch/builds/404317084
build duration: 372s

[rudyfly]

LGTM