Support per-file blocks-over-objects

[modeswitch]

Currently each file is stored in a single object. In some cases this can cause lots of unnecessary reads/writes. This can be fixed by assigning a per-file block size and converting the data pointer to a list of pointers to fixed-size blocks.

(I marked this as good first bug, but this work is somewhat more involved than other bugs with that label.)

[modeswitch]

I think we will need this to support large files. @humphd reported some issues with files larger than 33M.

[humphd]

http://pbouianov.wordpress.com/2014/02/03/project-update-wall/ is where @pbouianov was writing about this, fwiw.

[modeswitch]

The dropbox-style sync stuff is bigger than one issue. I'd like to spend some time scoping it out and we can split it into smaller tasks.

[modeswitch]

Bumping to milestone 2.