client: Fixed Visual Studio compiler warnings. In particular:

- Changed off_t type to int64_t in interface.
- Using size_t as index type where necessary.
- Added static_cast where signed and unsigned integers are used together.
- Replaced boost::*int_t* types with those from stdint.h

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3585 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534