Allow sync of large files

There's a pull request that will implement some of the code needed for this: https://github.com/andrejb/soledad/commit/4787f69b623e7533e1f2cc193b200115246bc46b

There are also some issues that will solve some of bigcouch's limitations by patching it: #4906, #4932.

This issue should track the implementation of large files sync in Soledad.

(from redmine: created on 2014-01-20, closed on 2014-03-18)