The motivation for this is to simplify formatters so
that they know they can store state in their instance
without fear that a Datastore is going to reuse the
Formatter for some other location.

* This changes the read() and write() methods since they no
  longer need FileDescriptor parameter.
* All the formatters need to be tweaked.
* Datastore needs changing to ensure that FileDescriptor is
  created a bit earlier.
* getFormatter gets an extra argument which can be None for
  formatters that will not be using read/write at all.

Instance retrieval is now layered on top of class retrieval.

This can be a class method and so be distinct from the instance
methoer predictPath