client: Backported revision 2968. Original commit message was:

mno-async_writes/client:
- Disabled logging of line and file by default (was always ?:0).
- Set minimum width of logged thread address to ensure that the beginning of all
logged lines has the same length.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3048 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534