retry mechanism in downloader

[CXwudi]

notes for retry in downloaders: (TODO)

• if one downloader failed, try the next downloader on the same PV
  • allow users to choose multiple downloaders for one PV service
  • allow us to try the faster risky one, if not successful, use the safer slower one
  • make sure that the global retry count is independent for each downloader
• if one PV service failed, try the next PV service
  • not all the time that VocaDB info of PV availability can be up to date, so we need our side to handle it
• if all original PVs from all services are failed, try download reupload PVs
  • this could be useful if a Vocaloid producer deleted all his/her contents, but other ppl upload a copy
• all retry features are configurable
  • how to resolve PV preference sorting base on current enabled retry features
  • An: sort and filter PVs base of all setting aboves

[CXwudi]

so we have 5 spring properties and each one has been well documented.
Brainstorming of this issue is pretty much done