a stream-like object for manipulating a compound key shelf file
that logs key modification times TreeShelf
Detail:
provides a mechanism for reading/writing to a shelf file
"behind the scenes". The source argument to the constructor
is a tuple (shelf file name, shelf key tuple), where
shelf key tuple is a tuple of strings as required by the
CompoundKeyShelf base class.
Note that the .read method is inherited from the _Stream
class.