tests: Test on valid data on snapshots when the original files change.
servers: Log error if FileMetadata is missing for the deletion stage.

For COW enabled files, the FileMetadata could be null and resulting in an
unhandled NullPointerException.
servers: Keep snapshot data if a file is deleted (fixes #325).

In case of delete operations files will be closed from the OpenFileTable
explicitly. Versioning, deletion etc. will be handled by EventCloseFile.
OpenFileTableEntry had to be extended to enable the required changes.
docs: Fix parameter order for snapshot examples.
Merge pull request #327 from xtreemfs/jdillmann-snapshot_fixes

Fix #325: Keep snapshot data when a file is deleted