new transfer job queue

* added QueuedLoader
  - concurrent transfers (yadayada#7)
  - retry on error (disabled by default)
* retry_on decorator added for transfer functions (jobs)
* api: add multiple read/write callbacks api for ul/dl
* api: progress printing removed
* api: fix for resuming of incomplete downloads
* db conn thread check disabled
* single file progress wrapper FileProgress added
* progress aggregator MultiProgress added
* progress speed determination improved
* download behavior changed to skip existing files

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