download: operations on existing local files

[myfreeer]

• processBeforeDownload
  • do nothing (currently default)
  • log or warn
  • mark as shouldBeDiscardedFromDownload
  • skip and return void
• download
  • do nothing (currently default)
  • log or warn
  • request newer version with If-Modified-Since and check Last-Modified
  • read from disk
  • read from disk if not newer than remote
  • discard
  • continue to download if remote is larger, read from disk if else
• saveToDisk
  • overwrite (currently default)
  • log or warn
  • save if newer by checking Last-Modified
  • save with another name

What about urls with savePath pointing at non-existing files redirected to existing files?
Maybe we can use a bitset containing options and 3 optional life cycle function for this.

[myfreeer]

This could be made into a optional life cycle callback, with all conditions passed in and operations as return value, the existance of local file are only detected if the life cycle exists.

[myfreeer]

read from disk option can cause side effects since links could be transformed, so it's out of the plan now