Merge CorvusCorax/treefiles

Merge CorvusCorax/treefiles

Adds support for exporting a single export as a swathload of files. This
is similar to, but not exactly the same as, the multifile export mode.
The difference lies in the fact that multifile expects the files to be
of differing sizes, and will not remove them; in contrast, the treefiles
mode will expect them all to be a single page in size, and will remove
them on a TRIM command.

This mode is useful for FUSE-backed exports which may have their storage
on a remote server (e.g., SSHFS), where it would be inefficient to
transfer the entire export for every (small) write.

* CorvusCorax/treefiles:
  added test cases for tree mode
  got rid of CamelCase
  modified documentation to reflect removal of -t command line switch
  Changed treefile implementation for better code compatibility
  removed "treefiles" option from commandline as commandline is deprecated by policy and no new features must be added
  removed new documentation section which had accidently been added twice
  fixed error handling, messages, cleanups
  fixed typo
  Fix mistake in trim address calculation
  Added support for "treefile" representation with one file per block in a subdir tree structure