[client] Fix transfer rate display.

* Display rate of the last second instead of this second.
* Display 0% 0KB/s in repo.html.
[http] Fix block handle leak when downloading file.
[Mac] Normalize filenames before stroing it into index.

On Mac OS X with HFS+,
* file system APIs accept both NFC and NFD encoded paths;
* readdir() returns dname in NFD.

Internally, Seafile assumes all filenames are in UTF-8, NFC.

So after reading dname from filesystem, we should normalize
it to NFC.
[http] Check for duplicate upload progress id.