Support preheat when using registry as backend

[mmpei]

@yiranwang52

[CLAassistant]

All committers have signed the CLA.

[codygibb]

Would you mind updating the PR description with a change summary, including:

• What is changing?
• Why is it changing?

[mmpei]

#145
the origin should cache blobs when using registry as backend.
now kraken only support pushing to registry directly and this PR will make origin cache the blobs of pushed images.
also it will works well when integrating with other image repository like harbor.

changes:
the proxy will listen on an addtional port for registry notification events. and it will pull the manifest and blob metaInfos as soon as receving a manifest pushing event. and this will make origin cache the blobs locally and reduce the time of process of P2P pulling.
@codygibb

[codygibb]

Overall the change looks pretty reasonable to me. I'm fine holding off on tests until the review stabilizes, but we should add some before this lands.

[mmpei]

@codygibb i have updated as your comment. please check again.
if it looks ok, i will do more test on this feature and raise a new PR with only one commit later.

[codygibb]

Content looks good, just needs some clean up. Let's add some tests (I think agent/agentserver/server_test.go is a good example to work off of).

[mmpei]

additional, i used to think the http server in proxy is mandatory.
making it to optional is due to the python UTs failed and config compatible.
and what is your opinion? @codygibb

[codygibb]

additional, i used to think the http server in proxy is mandatory.
making it to optional is due to the python UTs failed and config compatible.
and what is your opinion? @codygibb

Agreed. Let's keep it optional in this diff, but I'll make it required in a follow up diff and fix anything that breaks.

[mmpei]

it seems the fossa makes build failed. i have modified PR by your comments. and test is OK too including the log format. i will add some UT and raised a new PR later.

[codygibb]

it seems the fossa makes build failed. i have modified PR by your comments. and test is OK too including the log format. i will add some UT and raised a new PR later.

Yeah the fossa integration is broken right now. Feel free to ignore for now.

[codygibb]

Tests look great! Thank you for putting this together!