mno-async_writes/client:

- Abort when an AsyncWriteHandler has pending requests or observers left.
- Added assert() to ensure the maximum number of pending requests is not
  reached.


git-svn-id: https://xtreemfs.googlecode.com/svn/branches/mno-async_writes@3128 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534