client:

- Added option --max-writeahead-requests.
- Removed WriteAsync() from interface again. If async writes are enabled, is now determined by a bool in FileHandleImplementation.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2587 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534