add help doc for rd-api

- update "debug opts" to use putStrLn. no stupid quotes around string.
- add retry support for fetchBlockFromHttp (--block-max-retry)
- check http status code
  expect 200 on GET /rd/file.
  expect 206 on GET /file with Range header.
- handle http exceptions
- handle IOError at critical places.
- on client tool, use MaybeT to do early return, avoid shifting to the right.
- fix -f -v wrong option position in parser.
- other small fixes.

- enable -Wall -Werror in package.yaml
  fix issues
- support "pending" blocks. client will do incrementally download until
  all blocks are ready on server side and fetched to local disk.
- bugfix: fileWorker missing "forever" loop.
- moved genBlocks to RD.Lib

bugfix: worker should close file when done

sha1sum works fine with lazy ByteString.